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  • Oral Glucose Tolerance Test (OGTT)

    The OGTT is more reliable when the patient is ambulatory and does not have other severe acute or chronic illnesses. The test should be preceded by at least 3 days of adequate carbohydrate diet and should be performed in the morning after the patient has fasted at least 10 hours (but no longer than 16 hours). The test dose has been standardized by the NDDG at 75 gm of glucose or dextrose for nonpregnant persons and 100 gm for pregnant women. The dose may be calculated from body weight. Various ready-made commercial preparations can be used, or the dose can be given in up to 300 ml of water, usually flavored with a substance such as lemon juice. The dose should be consumed by the patient within 5 minutes. The test officially begins when the patient begins to drink. The NDDG recommends that the patient should remain seated during the test and should not smoke. One should also beware of medication that could affect test results, such as oral contraceptives, steroids, diuretics, and anticonvulsants.

    NDDG test protocol. Blood specimens are taken fasting, then every 30 minutes for 2 hours after the beginning of dextrose ingestion. After ingestion of the test dose, a lag period occurs, after which the blood glucose curve rises sharply to a peak, usually in 15-60 minutes. In one study, 76% had maximal values at 30 minutes and 17% at 1 hour. The curve then falls steadily but more slowly, reaching normal levels at 2 hours. These may be fasting (FBG) values or simply within the blood glucose reference range.

    Occasionally, after the fasting level is reached, there may follow a transient dip below the fasting level, usually not great, then a return to fasting values. This relative hypoglycemic phase of the curve (when present) is thought to be due to a lag in the ability of the liver to change from converting glucose to glycogen (in response to previous hyperglycemia) to its other activity of supplying glucose from glycogen. In some cases, residual insulin may also be a factor. This “hypoglycemic phase,” if present, is generally between the second and fourth hours. Several reports indicate that so-called terminal hypoglycemia, which is a somewhat exaggerated form of this phenomenon, occurs in a fairly large number of patients with a GTT response indicative of mild diabetes. They believe that an abnormally marked hypoglycemic dip often appears in mild diabetics 3-5 hours after meals or a test dose of carbohydrates and may be one of the earliest clinical manifestations of the disease.

    Flat oral glucose tolerance test. This is an OGTT the peak of which has been defined variously as less than 40 mg, 25 mg, or 20 mg/100 ml above the FBG value. The most commonly used definition is 25 mg/100 ml (1.38 mmol/L). The condition most frequently associated with a flat OGTT is small intestinal carbohydrate malabsorption due to sprue or celiac disease, with a lesser number of cases seen in myxedema and some cases reported in pituitary insufficiency. Flat OGTT results may also occur in clinically normal persons. When the 40 mg definition was used, one study reported a flat OGTT in 90% of patients with sprue, but also in up to 40% of clinically normal persons. When the 25 mg definition was used, another study reported a flat OGTT in about 60% of patients with sprue. Using the 20 or 25 mg definition, several investigators found a flat OGTT result in about 20% of clinically normal persons (range, 7%-25%).

    Oral glucose tolerance test interpretation. In the past, criteria for interpretation of the OGTT have varied widely. This situation is brought about because of the absence of a sharp division between diabetics and nondiabetics, variations in methodology, and variations in adjustment for the many conditions that may affect the GTT quite apart from idiopathic diabetes mellitus; some of these factors have been mentioned previously, and others will be discussed later. The NDDG criteria are rapidly replacing previous criteria as world-recognized standards. The NDDG criteria are listed in Table 28-2. Please note that all values from now on in the text discussion will be given in milligrams per 100 ml, using true glucose methods unless otherwise stated.

    28-2

    Table 28-2 National diabetes data group criteria for diagnosis of diabetes mellitus in nonpregnant and pregnant adults*

    The NDDG has permitted a diagnosis of diabetes mellitus to be made in any one of three different ways:

    1. Sufficient classical symptoms of diabetes mellitus (e.g., polydipsia, polyuria, ketonuria, and weight loss) plus either an unequivocal elevation of the fasting glucose (FBG) level or an elevation of the non-FBG level greater than 200 mg/100 ml (11.1 mmol/L).
    2. Elevation of the FBG level (venous serum or plasma) greater than 140 mg/100 ml (7.8 mmol/L) on more than one occasion (assuming no condition is present that falsely increases blood glucose values).
    3. A normal FBG level but OGTT peak and 2-hour values both greater than 200 mg/100 ml (11.1 mmol/L) on more than one occasion.

    Three points should be noted. First, the diagnosis of diabetes can be made in nonpregnant adults if typical clinical symptoms are present plus a nonfasting serum specimen more than 200 mg/100 ml. Second, the diagnosis can be made without requiring a GTT if the FBG level is sufficiently elevated. Third, when the diagnosis is based predominantly on blood glucose measurement, either the FBG level or the OGTT, diagnosis requires sufficient abnormality on 2 different days rather than only one occasion.

    Impaired glucose tolerance. The NDDG recognizes a category of OGTT curve that it calls “impaired” glucose tolerance, which is significantly abnormal values but not sufficiently abnormal to make a diagnosis of diabetes (Table 28-3). This involves an FBG level less than 140 mg/100 ml (7.77 mmol/L) and a single point on the OGTT curve at or above 200 mg/100 ml (11.1 mmol/L); that is, either the peak or the 2-hour value greater than 200 mg/100 ml, but not both.

    28-3

    Table 28-3 National diabetes data group classification of glucose tolerance abnormalities

    There are abnormal areas in the OGTT that include FBG between 115-140 mg/100 ml (6.4-7.8 mmol/L) and other points above reference range but less than 200 mg/100 ml (11.1 mmol/L); in other words, intermediate between normal and impaired OGTT. The NDDG calls these “nondiagnostic abnormalities.” World Health Organization (WHO) 1980 criteria for diagnosis of diabetes mellitus and for impaired OGTT are the same as those of the NDDG. However, WHO considers the NDDG category of nondiagnostic abnormalities as being normal, with the exception of a 2-hour value between normal and 200 mg/100 ml (11.1 mmol/L), which WHO includes in the impaired OGTT category.

    Oral glucose tolerance test criteria for children. In children, criteria for diagnosis of diabetes mellitus are rather similar to those of nonpregnant adults, but there are a few significant differences. First, if the child has classic symptoms of diabetes, a single random nonfasting serum glucose value at or above 200 mg/100 ml (11.1 mmol/L) is sufficient for diagnosis. Second, the upper limit of the FBG reference range is set at 130 mg/100 ml (7.2 mmol/L) instead of the nonpregnant adult upper limit of 115 mg/100 ml (6.4 mmol/L). However, FBG values necessary for diagnosis of diabetes are the same for children and adults (140 mg/100 ml; 7.8 mmol/L). Second, the glucose dose for children is calculated on the basis of patient weight (1.75 gm/kg of ideal weight to a maximum of 75 gm). Third, elevation of the FBG level alone is not sufficient in children. The FBG level must be more than 140 mg/100 ml (7.8 mmol/L) and either the peak or the 1-hour value must be more than 200 mg/100 ml (11.1 mmol/L) (if the FBG level is normal, both the peak and the 2-hour value must be more than 200 mg/100 ml, which is the same requirement listed for adults).

    Gestational diabetes

    The NDDG definition of gestational diabetes is abnormal glucose tolerance with onset or recognition during pregnancy but not before. Six weeks or more after the end of pregnancy, the patient should be retested using the standard nonpregnant OGTT with standard nonpregnant NDDG criteria and reclassified into previous abnormal glucose tolerance (if the postpartum standard nonpregnant OGTT result is normal), impaired glucose tolerance (if the standard nonpregnant OGTT result is abnormal but not sufficiently abnormal to fit the NDDG criteria for diabetes), or diabetes mellitus. The American Diabetes Association (1986) recommends that pregnant women who are not known to have abnormal glucose tolerance should have a special screening test between the 24th and 28th week consisting of a test dose of 50 gm of oral glucose (fasting or nonfasting). A single postdose 1-hour venous plasma value of 140 mg/100 ml (7.8 mmol/L) or more suggests the need for the full NDDG gestational OGTT during the pregnancy. The gestational OGTT consists of FBG, 1-hour, 2-hour, and 3-hour specimens following a 100-gm oral glucose dose.

    There is general agreement about acceptability of the gestational 50-gm 1-hour screening test. However, one investigator has reported 11% more abnormal results when the 50-gram test was performed at 28 weeks’ gestation than when it was performed on the same patients at 20 weeks, and an additional 8% results were abnormal at 34 weeks than at 28 weeks. There is significantly more controversy regarding the NDDG gestational 100-gm 3-hour diagnostic test. In one study, 17% of patients who had initially normal NDDG 3-hour test results had one abnormal value when a repeat test was done 1-2 weeks later, and 5% had initially abnormal results that were normal when the test was repeated. Overall, it appears that about 25% of initial gestational NDDG 3-hour test results will significantly change when the test is repeated. However, the greatest controversy involves the glucose levels selected as cutoff points between normal and abnormal. This controversy arises because several investigators have found nearly as many newborns with macrosomia (about 30%) in mothers having one significantly elevated time point on the NDDG 3-hour gestational OGTT as in those mothers who had two significantly elevated time points (required for diagnosis of diabetes). This suggested that the gestational NDDG criteria for diabetes were too conservative. Several investigators have proposed revisions of the NDDG gestational 3-hour OGGT criteria; the two most frequently cited in the literature are shown in Table 28-4. One recent report found that about half of study patients with one significantly elevated time point on the NDDG 3-hour OGTT had two significantly elevated time points (therefore, diagnostic of diabetes) using the Carpenter-modified OGGT criteria; about half these mothers had macrosomic infants and half did not.

    28-4

    Table 28-4 Revised cutoff points proposed for abnormality in the 100-gm 3-hour diagnostic test for gestational diabetes

    Screening tests for diabetes

    Screening tests for diabetes attempt to circumvent the multiple blood glucose determinations required for the GTT. The FBG level and the 2-hour postprandial (2 hours after a meal) blood glucose level have been widely used. Since these essentially are isolated segments of the GTT curve, interpretation of their results must take several problems into consideration in addition to those inherent in the GTT.

    An abnormality in the FBG level for example, raises the question of whether a full GTT is needed for confirmation of diabetes. Most authorities, including the NDDG panel, believe that if the FBG level is sufficiently elevated, there is no need to do the full GTT. Whatever the etiology of the abnormal FBG level, the GTT result will also be abnormal. Since it is known in advance that the GTT result will be abnormal, no further information is gained from performing the GTT. Most also agree that a normal FBG value is not reliable in ruling out possible diabetes. In one study, 63% of those with diabetic GTT results had normal FBG values. Others have had similar experiences, although perhaps with less striking figures.

    Most investigators believe that of all the postprandial GTT values, the 2-hour level is the most crucial. The 2-hour value alone has therefore been proposed as a screening test. This recommendation is based on the fact that with a normal FBG level, the diagnosis of diabetes mellitus cannot be made with confidence on the basis of an abnormal peak blood glucose level if it is accompanied by a normal 2-hour blood glucose level in the OGTT. The main reason for this lies in the effect of gastric emptying on glucose absorption. It has been fairly well proved that normal gastric emptying does not deliver a saturation dose to the duodenum. Therefore, slow gastric emptying tends to produce a low, or “flat,” GTT curve. On the other hand, either unusually swift gastric transit or delivery of a normal total quantity of glucose to the small intestine within a markedly shortened time span results in abnormally large amounts of glucose absorbed during the initial phases of the tolerance test. Since homeostatic mechanisms are not instantaneous, the peak values of the tolerance curve reach abnormally high figures before the hyperglycemia is brought under control. An extreme example of this situation occurs in the “dumping syndrome” produced by gastrojejunostomy.

    If previous time interval specimens are considered unreliable, the question then is justified as to whether the 2-hour value alone is sufficient for diagnosis of diabetes. According to the NDDG criteria, both the peak level and the 2-hour level must be greater than 200 mg/100 ml (11.1 mmol/L), so that a full GTT is necessary, even though it is uncommon to find a 2-hour value more than 200 mg/100 ml and a peak value less than 200 mg/100 ml. The NDDG recommendation may be based on reports that one variant of the normal OGTT drops relatively swiftly to normal at approximately 1.5 hours and then rebounds above 140 mg/100 ml (7.8 mmol/L) by 2 hours. In one report this phenomenon occurred in as many as 5%-10% of patients.

    Glucose tolerance in other diseases

    Besides the intrinsic and extrinsic factors that modify response to the OGTT, other diseases besides diabetes mellitus regularly produce diabetic-type GTT patterns or curves. Among these are adrenal, thyroid, and pituitary hormone abnormalities that influence liver or tissue response to blood glucose levels. Cushing’s syndrome results from hypersecretion of hydrocortisone. Since this hormone stimulates gluconeogenesis among its other actions, 70%-80% of patients with Cushing’s syndrome exhibit decreased carbohydrate tolerance, including 25% who exhibit overt diabetes. One report suggests that patients with gestational diabetes who are in the third trimester have fasting plasma cortisol levels higher than those of nonpregnant women. Pheochromocytomas of the adrenal medulla (or elsewhere) have been reported to produce hyperglycemia in nearly 60% of affected patients and glucosuria in a lesser number. These tumors produce norepinephrine or epinephrine, either continually or intermittently. The diabetogenic effects of epinephrine were mentioned earlier, and it has been noted that pheochromocytomas that secrete norepinephrine rather than epinephrine are not associated with abnormalities of carbohydrate metabolism. Primary aldosteronism leads to the overproduction of aldosterone, the chief electrolyte-regulating adrenocortical hormone. This increases renal tubular excretion of potassium and retention of sodium. Patients with primary aldosteronism frequently develop decreased carbohydrate tolerance. According to Conn, this is most likely due to potassium depletion, which in some manner adversely affects the ability of pancreatic beta cells to respond normally to a hyperglycemic stimulus. Parenthetically, there may be some analogy in reports that chlorothiazide diuretics may cause added decrease in carbohydrate tolerance in diabetics, thus acting as diabetogenic agents. Some say that no such effect exists without some degree of preexisting glucose tolerance abnormality, but others maintain that it may occur in a few clinically normal persons. Chlorothiazide often leads to potassium depletion as a side effect; indeed, one report indicates that potassium supplements will reverse the diabetogenic effect. However, other mechanisms have been postulated.

    Thyroid hormone has several effects on carbohydrate metabolism. First, thyroxine acts in some way on small intestine mucosal cells to increase hexose sugar absorption. In the liver, thyroxine causes increased gluconeogenesis from protein and increased breakdown of glycogen to glucose. The metabolic rate of peripheral tissues is increased, resulting in an increased rate of glucose utilization. Peripheral tissue glycogen is depleted. Nevertheless, the effect of hyperthyroidism on the GTT is variable. Apparently the characteristic hyperthyroid curve is one that peaks at an unusually high value, sometimes with glucosuria, but that returns to normal ranges by 2 hours. However, in one extensive survey, as many as 7% of hyperthyroid patients were reported to have diabetic curves, and another 2% had actual diabetes mellitus. Surprisingly, the type of curve found in any individual patient was not related to the severity of the hyperthyroidism. In myxedema, a flat OGTT curve (defined as a peak rise of <25 mg/100 ml [1.4 mmol/L] above the FBG value) is common. However, since absorption defects vary in degree and are counterposed against decreased tissue metabolism, one investigator reported that 50% of hypothyroid patients tested had a diabetic type of OGTT curve with the FBG value usually normal.

    Acromegaly is reported to produce an elevated FBG level in 25% of cases and a diabetic OGTT curve in 50% of cases. Growth hormone (somatotropin) is thought to be able to stimulate gluconeogenesis independently. Actually, the influence of the pituitary on carbohydrate metabolism has been mainly studied in conditions of pituitary hypofunction; in hypopituitarism, a defect in gluconeogenesis was found that was due to a combination of thyroid and adrenocorticosteroid deficiency rather than to deficiency of either agent alone.

    In acute pancreatitis, perhaps 25%-50% of patients may have transient hyperglycemia. In chronic pancreatitis, abnormal glucose tolerance or outright diabetes mellitus is extremely common.

    A variety of nonendocrine disorders may produce diabetogenic effects on carbohydrate tolerance. Chronic renal disease with azotemia frequently yields a diabetic curve of varying degree, sometimes even to the point of fasting hyperglycemia. The reason is not definitely known. Hyperglycemia with or without glucosuria occurs from time to time in patients with cerebral lesions, including tumors, skull fracture, cerebral infarction, intracerebral hemorrhage, and encephalitis. The mechanism is not known, but experimental evidence suggests some type of center with regulatory influence on glucose metabolism located in the medulla and the hypothalamus, and perhaps elsewhere. Similar reasoning applies to the transient hyperglycemia, sometimes accompanied by glucosuria, seen in severe carbon monoxide poisoning. This is said to appear in 50% of these patients and seems to be due to a direct toxic effect on the cerebral centers responsible for carbohydrate metabolism. Type IV lipoproteinemia is frequently associated with some degree of decreased carbohydrate tolerance, which sometimes may include fasting hyperglycemia.

    Malignancies of varying types are reported to produce decreased carbohydrate tolerance in varying numbers of patients, but the true incidence and the mechanism involved are difficult to ascertain due to the presence of other diabetogenic factors such as fever, cachexia, liver dysfunction, and inactivity.

    Liver disease often affects the OGTT response. This is not surprising in view of the importance of the liver in carbohydrate homeostasis. Abnormality is most often seen in cirrhosis; the degree of abnormality has a general (although not exact) correlation with degree of liver damage. In well-established cirrhosis, the 2-hour postprandial blood glucose level is usually abnormal. The FBG level is variable but is most often normal. Fatty liver may produce GTT abnormality similar to that of cirrhosis. In hepatitis virus infection there is less abnormality than in cirrhosis; results become normal during convalescence and may be normal at all times in mild disease.

    Acute myocardial infarction has been shown to precipitate temporary hyperglycemia, glucosuria, or decreased carbohydrate tolerance. In one representative study, 75% of patients had abnormal GTT responses during the acute phase of infarction, with 50% of these being frankly diabetic curves; follow-up showed that about one third of the abnormal curves persisted. Besides the well-known increased incidence of atherosclerosis (predisposing to infarction) in overt or latent diabetics, emotional factors in a stress situation and hypotension or hepatic passive congestion with liver damage may be contributory.

    Emotional hyperglycemia is considered a well-established entity presumably related to epinephrine effect.

    OGTT responses in pregnancy were discussed earlier. Some investigators believe that the OGTT curve in gravid women does not differ from that in nulliparas, and thus any abnormalities in the curve are indicative of latent diabetes. Others believe that pregnancy itself, especially in the last trimester, tends to exert a definite diabetogenic influence (controversy here concerns how much change is considered “normal.” This view is reinforced by observations that the original synthetic estrogen-progesterone combinations used for contraception (with higher estrogen content than most current types) often mimicked the diabetogenic effects of pregnancy. This occurred in 18%-46% of patients, and, as in pregnancy, the FBG level was most often normal. The exact mechanism is not clear; some suggest altered intestinal absorption.

    Salicylate overdose in children frequently produces a clinical situation that closely resembles diabetic acidosis. Salicylate in large quantities has a toxic effect on the liver, leading to decreased glycogen formation and to increased breakdown of glycogen to glucose. Therefore, there may develop a mild to moderate elevation in blood glucose level, accompanied by ketonuria. Plasma ketone test results may even be positive, although usually only to mild degree. Salicylic acid metabolites give positive results on tests for reducing substances such as Clinitest, so that results of such tests falsely suggest glucosuria. In addition, salicylate stimulates the central nervous system (CNS) respiratory center in the early phases of overdose, so that increased respiration may suggest the Kussmaul breathing of diabetic acidosis. The carbon dioxide (CO2) content (or partial pressure of CO2, (PCO2) is decreased. Later on, a metabolic acidosis develops.

    Differentiation from diabetic acidosis can be accomplished by simple tests for salicylate in plasma or urine. A dipstick test called Phenistix is very useful for screening purposes. A positive plasma Phenistix reaction for salicylate is good evidence of salicylate poisoning. A positive result in urine is not conclusive, since in urine the procedure will detect nontoxic levels of salicylate, but a negative urine result is strong evidence against the diagnosis. Definitive chemical quantitative or semiquantitative tests for blood salicylate levels are available. It is important to ask about a history of medication given to the patient or the possibility of accidental ingestion in children with suspicious clinical symptoms.

    Salicylate intoxication is not frequent in adults. When it occurs, there is much less tendency toward development of a pseudodiabetic acidosis syndrome. In fact, in adults, salicylate in nontoxic doses occasionally produces hypoglycemia, which tends to occur 2-4 hours postprandially.

    Phenytoin (Dilantin) is reported to decrease glucose tolerance, and overdose occasionally produces a type of nonketotic hyperglycemic coma.

    Posthypoglycemic hyperglycemia (Somogyi phenomenon) refers to fasting hyperglycemia produced by hormonal (epinephrine, catecholamines, growth hormone) response to previous hypoglycemia induced by too much administered insulin. This can produce the false appearance of treatment failure due to insufficient insulin.

    Dawn phenomenon refers to circadian increase in plasma insulin levels between 3 A.M. and 7 A.M. This occurs in response to increased plasma glucose level produced by circadian increase in pituitary growth hormone secretion. In nondiabetic persons the plasma glucose level remains normal in response to the insulin. In diabetics, including those who are insulin dependent and some who are noninsulin dependent, impaired glucose tolerance permits plasma glucose levels to become elevated to some degree over baseline values during this time. In some patients the 6 A.M. or 7 A.M. fasting glucose level becomes sufficiently elevated to simulate necessity for higher doses of daily insulin, whereas more insulin is needed only during this limited time period.

    Controversy on clinical relevance of the oral glucose tolerance test

    Complete discussion of the OGTT must include reference to various studies that attack the clinical usefulness of the procedure. These consist of reports of large series of normal persons showing up to 20% flat GTT results, studies that showed different curves in repeat determinations after time lapse, and others in which various types of curves were obtained on repeated tests in the same individual. Based on the criteria in use before the NDDG recommendations, some investigators believed there is inadequate evidence that GTT abnormality actually indicates true diabetes mellitus, since many persons with abnormal GTT responses failed to progress to clinical diabetes, and the population incidence of diabetes was far short of that predicted by GTT screening. Since the NDDG criteria require somewhat more abnormality than previous criteria to make a diagnosis of diabetes mellitus, these problems have been partially corrected. Even so, not all drawbacks of the OGTT have been solved regarding problems of sensitivity, specificity, reproducibility, and clinical relevance even when the test is performed under optimal conditions. Nevertheless, at present the OGTT is still the standard test of carbohydrate tolerance and the laboratory basis for the diagnosis of diabetes mellitus.

  • Glucose Tolerance Test

    The diagnosis of diabetes is made by demonstrating abnormally increased blood glucose values under certain controlled conditions. If insulin deficiency is small, abnormality is noted only when an unusually heavy carbohydrate load is placed on the system. In uncompensated insulin deficiency, fasting glucose is abnormal; in compensated insulin deficiency, a variety of carbohydrate tolerance test procedures are available to unmask the defect. To use and interpret these procedures, one must thoroughly understand the various factors involved.

    Glucose tolerance tests (GTTs) are provocative tests in which a relatively large dose of glucose challenges the body homeostatic mechanisms. If all other variables are normal, it is assumed that the subsequent rise and fall of the blood glucose is due mainly to production of insulin in response to hyperglycemia and that the degree of insulin response is mirrored in the behavior of the blood glucose. Failure to realize that this assumption is predicated on all other variables being normal explains a good deal of the confusion that exists in the literature and in clinical practice.

    Test standardization

    The most important factor in the GTT is the need for careful standardization of the test procedure. Without these precautions any type of GTT yields such varied results that an abnormal response cannot be interpreted. Previous carbohydrate intake is very important. If diet has been low in both calories and carbohydrates for as little as 3 days preceding the test, glucose tolerance may be diminished temporarily and the GTT may shift more or less toward diabetic levels. This has been especially true in starvation, but the situation does not have to be this extreme. Even a normal caloric diet that is low in carbohydrates may influence the GTT response. A preparatory diet has been recommended that includes approximately 300 gm of carbohydrates/day for 3 days preceding the test, although others believe that 100 gm for each of the 3 days is sufficient. The average American diet contains approximately 100-150 gm of carbohydrates; it is obviously necessary in any case to be certain that the patient actually eats at least 100 gm/day for 3 days.

    Factors that affect the glucose tolerance test

    Inactivity has been reported to have a significant influence on the GTT toward the diabetic side. One study found almost 50% more diabetic GTT responses in bedridden patients compared with ambulatory patients identical in most other respects. The effect of obesity is somewhat controversial. Some believe that obesity per se has little influence on the GTT. Others believe that obesity decreases carbohydrate tolerance; they have found significant differences after weight reduction, at least in obese mild diabetics. Fever tends to produce a diabetic-type GTT response; this is true regardless of the cause but more so with infections. Diurnal variation in glucose tolerance has been reported, with significantly decreased carbohydrate tolerance during the afternoon in many persons whose GTT curves were normal in the morning. This suggests that tests for diabetes should be done in the morning. Stress, when severe, results in release of various hormones (e.g., epinephrine and possibly cortisol and glucagon), which results in decreased glucose tolerance. Acute myocardial infarction, trauma, burns, and similar conditions frequently are associated with transient postprandial hyperglycemia and occasionally with mild fasting hyperglycemia. This effect may persist for some time. It has been recommended that definitive laboratory testing for diagnosis of diabetes be postponed for at least 6 weeks. However, if the fasting blood glucose (FBG) level is considerably elevated and there is substantial clinical evidence of diabetes, the diagnosis can be made without additional delay.

    There is a well-recognized trend toward a decreasing carbohydrate tolerance with advanced age. For each decade after age 30, fasting glucose increases 1-2 mg/100 ml (0.05-0.10 mmol/L) and the 2-hour value increases 8-20 mg/100 ml (0.4-1.1 mmol/L). There are three schools of thought as to the interpretation of this fact. One group believes that effects of aging either unmask latent diabetes or represent true diabetes due to impairment of islet cell function in a manner analogous to subclinical renal function decrease through arteriosclerosis. Another group applies arbitrary correction formulas to decrease the number of abnormalities to a predetermined figure based on estimates of diabetes incidence in the given population. A third group, representing the most widely accepted viewpoint, regards these changes as physiologic rather than pathologic. To avoid labeling many elderly persons diabetic who have no other evidence of diabetes, some experts deliberately extend the upper limits of the oral GTT reference range. The National Diabetes Data Group (NDDG) diabetes criteria (discussed later) incorporate some of this shift of the reference range.

    The question arises occasionally as to what serum glucose values are normal when a patient is receiving intravenous 5% dextrose. In 20 patients at our hospital who had no evidence of disease known to affect serum glucose, values ranged from 86-232 mg/100 ml (4.74-12.78 mmol/L), with a mean value of 144 mg/100 ml (8.0 mmol/L). Only one patient exceeded 186 mg/100 ml (103 mmol/L).

  • Methods of Blood Glucose Assay

    The technique of blood glucose determination must be considered because different methods vary in specificity and sensitivity to glucose. The blood specimen itself is important; according to several reports (and my own experience), during each hour of standing at room temperature, whole blood glucose values decrease about 10 mg/100 ml unless a preservative is added. A high hematocrit value accentuates glucose decrease due to RBC metabolic activity. Fluoride is still the most recommended preservative. Plasma and serum are more stable than whole blood. If serum can be removed from the cells before 2 hours, serum glucose values remain stable for up to 24 hours at room temperature (although some authors report occasional decreases). Refrigeration assists this preservation. Serum or plasma values are generally considered to be 10%-15% higher than those of whole blood. However, several studies have reported considerable variation, ranging from 3% to 47%, in this difference over periods of time. Most current automated equipment use serum. Some small whole-blood office-type or portable analyzers are available, either single-test dedicated instruments (e.g., Yellow Springs glucose analyzer), reagent cartridge type (e.g., Abbott Vision or HemoCue-BG), or reagent strip types (Kodak Ektachem or Bohringer Reflotron). Venous blood is customarily used for glucose measurement. Capillary (arterial) blood values are about the same as those for venous blood when the patient is fasting. Nonfasting capillary values, however, average about 30 mg/100 ml (1.6 mmol/L) higher than venous blood, and this difference may sometimes be as great as 100 mg/100 ml (5.55 mmol/L).

    Biochemical methods. There are a considerable number of methods for blood glucose determination. These may be conveniently categorized as nonspecific reducing substance methods, which yield values significantly above true glucose values (Folin-Wu manual method and neocuproine SMA 12/60 automated method); methods that are not entirely specific for glucose but that yield results fairly close to true glucose values (Somogyi-Nelson, orthotoluidine, ferricyanide); and methods using enzymes that are specific for true glucose (glucose oxidase and hexokinase). There are certain technical differences and interference by certain medications or metabolic substances that account for nonuniformity of laboratory methodology and that in some instances may affect interpretation. Reference values mentioned in this chapter are for serum and for true glucose unless otherwise specified.

    “Bedside” paper strip methods. Another test for glucose consists of rapid quantitative paper strip methods (Dextrostix, Visidex, Chemstrip-BG, and others) available from several manufacturers. A portion of the paper strip is impregnated with glucose oxidase, an enzyme specific for glucose, plus a color reagent. One drop of whole blood, plasma, or serum is placed on the reagent area, and the color that develops is compared with a reference color chart. Visidex has two reagent areas that correspond to low- and high-glucose value areas. Small electronic readout meters are available for several of the manufacturer’s paper strips. The meters have generally been reported to make a substantial improvement in accuracy. Evaluations of the various paper strip methods provide a consensus that, with experienced personnel and with the use of a readout meter, experiments using quality control material or glucose solutions generally agree with standard laboratory methods within about ±5%. Using actual patient fingerstick capillary blood specimens, values between 40 and 130 mg/100 ml (2.2-7.2 mmol/L) usually agree within about ±15% (range, 8%-40%) with values obtained by standard laboratory methods. Persons without much familiarity with the technique may obtain more erratic results. These paper strip methods have been used with venous whole blood or finger puncture blood as a fast way to diagnose hypoglycemia and hyperglycemia in comatose or seriously ill persons and to provide guidance for patient self-adjustment of insulin dosage at home.

    Some cautions include possible differences between capillary (finger puncture) blood and venous blood values, alluded to previously; and effects of hematocrit value on results, since blood with a low hematocrit value (<35%) produces a higher result (by about 10%-15%), whereas blood with a high hematocrit value (>55%) produces a lower result. This creates a special problem with newborns, who normally have a high hematocrit value compared to adults. Also, quality control or evaluation of different manufacturer’s products by using glucose solutions may not accurately predict results using patient blood specimens. Very high serum levels of ascorbic acid (vitamin C) or gross lipemia may interfere. Patients with hyperosmolar hyperglycemia, with or without ketosis, may show test strip results that are lower than true values. Capillary specimens from cyanotic areas or from patients in shock may produce falsely low results. In one study of patients in shock, 64% of patients had fingerstick levels over 20% less than venous ones, and 32% of patients had fingerstick levels over 50% less than venous ones.

  • Diabetes

    Besides secreting exocrine digestive enzymes into the duodenum, the pancreas has endocrine functions centered in the islands of Langerhans. These structures are found primarily in the tail and body of the pancreas, the hormones involved are glucagon and insulin, and secretion is directly into the bloodstream. Diabetes mellitus results from abnormality in the production or the use of insulin. Production abnormality involves the islet beta cells and can be of two types: deficient beta-cell insulin production, or relatively normal synthesis but abnormal release. Besides production abnormality, diabetes may result from extrapancreatic factors such as peripheral tissue cell receptor dysfunction producing resistance to the cellular action of insulin, or abnormalities of nonpancreatic hormones that affect insulin secretion or blood glucose metabolism.

    Categories of diabetics

    The two types of idiopathic islet cell insulin abnormalities are associated with two of the most important clinical categories of diabetics. The first is the type I, or insulin-dependent, category of the National Diabetes Data Group (NDDG). Type I diabetes usually (but not always) begins relatively early in life and is more severe. Patients require insulin for management and show severe insulin deficiency on blood insulin assay. The second type of diabetes mellitus is the NDDG type II, or noninsulin-dependent diabetes, affecting about 80% of diabetics. Type II diabetes usually (but not always) begins in middle age or afterward, is frequently associated with overweight body status, is associated with less severe blood glucose abnormality, and can be treated by diet alone, oral medication, or small doses of insulin. Some type II persons show significantly elevated or normal insulin production on insulin blood level assay but a decrease in liver and peripheral tissue insulin use (insulin resistance). Others have varying degrees of decreased insulin production, although usually not as severe as the insulin deficiency of textbook type I diabetics.

    There is a small subgroup of teen-aged diabetics who have disease resembling type II adult diabetes. A recent report links this to mutation in the gene for glucokinase. There are also a few adult diabetics with type II disease who are not overweight, and a small subgroup of adult diabetics who have disease resembling type I.

    The NDDG has two other categories of diabetics. The first group is associated with various nonidiopathic conditions and syndromes (“secondary diabetes”) that either destroy pancreatic tissue (pancreatitis, pancreatic carcinoma, hemochromatosis) or produce abnormal glucose tolerance due to various extrapancreatic influences such as hormones, drugs, and insulin receptor abnormalities. The second category is gestational diabetes, diabetes that begins in pregnancy.

    Laboratory tests for diabetes

    Most laboratory tests for diabetes attempt to demonstrate pancreatic islet cell malfunction, either deficient insulin production or abnormal insulin release, using either direct or indirect blood insulin measurement. For many years direct blood insulin measurement was technically too difficult for any but a few research laboratories. Therefore, emphasis in clinical medicine was placed on indirect methods, whose end point usually demonstrated the action of insulin on a relatively accessible and easily measurable substance, blood glucose. Immunoassay methods for insulin measurement are now commercially available. However, in most cases direct insulin assay has not proved more helpful than blood glucose measurement in the diagnosis of diabetes, since in general the quantitative result and the pattern of blood glucose values permit one to separate diabetics into the two basic type I and type II groups with a reasonable degree of accuracy. In addition, blood glucose measurement is far less expensive, more readily available, and less technically demanding than current immunoassay methods.

    For reasons already noted, blood glucose measurement is still the mainstay for diagnosis of diabetes. Unfortunately, certain flaws are inherent in all systems using blood glucose for this purpose. These problems derive from any technique that attempts to assay one substance by monitoring its action on another. Ideally, one should measure a substrate that is specific for the reaction or enzyme in question under test conditions that eliminate the effects on use by any other factors. The blood glucose level does not meet any of these criteria.

    Blood glucose regulation

    The blood glucose level depends primarily on the liver, which exerts its effect on blood glucose homeostasis via its reversible conversion of glucose to glycogen, as well as via gluconeogenesis from fat and protein. Next most important is tissue utilization of glucose, which is mediated by pancreatic insulin but is affected by many factors in addition to insulin.

    The actual mechanisms involved in the regulation of blood glucose levels are complex and in many cases only partially understood. Insulin is thought to increase glucose transport into cells of most tissues (except red blood cells [RBCs] and possibly brain and intestinal mucosa) and to stimulate glucose oxidation and synthesis of fat, glycogen, and protein. In addition, insulin has a direct effect on the liver by suppressing glucose formation from glycogen (glycogenolysis).

    The liver is affected by at least three important hormones: epinephrine, glucagon, and hydrocortisone (cortisol). Epinephrine from the adrenal medulla stimulates breakdown of glycogen to glucose by converting inactive hepatic cell phosphorylase to active phosphorylase, which mediates the conversion of glycogen to glucose-1-phosphate. In addition, there is evidence that gluconeogenesis from lactate is enhanced by the action of the enzyme adenosine 3,5-monophosphate. Glucagon is a hormone produced by the pancreatic alpha cells and released by the stimulus of hypoglycemia. It is thought to act on the liver in a manner similar to that of epinephrine. Cortisol, cortisone, and similar 11-oxygenated adrenocorticosteroids also influence the liver but in a different manner. One fairly well-documented pathway is enhancement of glycogen synthesis from amino acids. This increases the carbohydrate reserve available to augment blood glucose levels; thus, steroids like cortisol essentially stimulate gluconeogenesis. In addition, cortisol deficiency leads to anorexia and also causes impairment of carbohydrate absorption from the small intestine.

  • Cystic Fibrosis of the Pancreas

    A few words should be said about the diagnosis of cystic fibrosis, a hereditary disease carried by a recessive gene. Symptoms usually begin in childhood but may not be manifested until adolescence or occasionally not until adulthood. The disease affects the mucous glands of the body but for some reason seems to affect those of the pancreas more than other organs. The pancreatic secretions become thick and eventually block pancreatic acinar ducts, leading to secondary atrophy of the pancreatic cells. The same process may be found elsewhere; as in the lungs, where inspissated secretions may lead to recurrent bronchopneumonia; and in the liver, where thickened bile may lead to plugging of the small ducts and to a secondary cirrhosis in very severe disease. These patients usually do not have a watery diarrhea, but this is not always easy to ascertain by the history. The diagnosis is made because the sweat glands of the body are also involved in the disease. Although these patients excrete normal volumes of sweat, the sodium and chloride concentration of the sweat is much higher than in normal persons.

    Cystic fibrosis in children should be differentiated from celiac disease. Celiac disease is basically the childhood form of the nontropical sprue seen in adults, both of which in many cases seem due to hypersensitivity to gluten. Gluten is found in wheat, oats, and barley and causes both histologic changes and clinical symptoms that are indistinguishable from those of tropical sprue, which is not influenced by gluten. These patients have normal sweat electrolytes, often respond to a gluten-free diet, and behave as ordinary malabsorption syndrome patients.

  • Chronic Pancreatitis

    Chronic pancreatic insufficiency may occur as a result of pancreatitis or hemochromatosis in adults and in the disease known as cystic fibrosis of the pancreas in children. The diagnosis may be quite difficult, since the disease either represents an end-stage phenomenon with an acute process going on or else may take place slowly and subclinically over a long period. The classic case of chronic pancreatitis consists of diabetes, pancreatic calcification on x-ray study, and steatorrhea. The diagnosis of diabetes will be discussed in the next chapter. Either of these parameters may be normal or borderline in many patients. The sensitivity of this test, and that of other pancreatic function tests, depends on the amount and degree of pancreatic tissue destruction and whether it occurs acutely or in a low-grade fashion.

    Tests useful in diagnosis of chronic pancreatitis

    Serum amylase. The serum amylase level in chronic pancreatitis is important, although it is much less reliable than in acute disease. In about one half of the patients it is within normal range. Repeated determinations are necessary at intervals of perhaps 3 days. Moreover, the values may be borderline or only slightly elevated, leading to confusion with other causes of elevated amylase levels mentioned previously. In this situation, the urine amylase level or the A/CCR is the most helpful test.

    Serum immunoreactive trypsin. Serum immunoreactive trypsin was discussed earlier as a test for acute pancreatitis. In chronic pancreatitis with pancreatic insufficiency there is variation in SIT data, depending on the severity of deficiency. In severely deficient cases 75% or more patients are reported to have decreased SIT values. In mild or moderate cases the values are more often within reference limits. One report indicated decreased values in nearly 40% of childhood diabetics on insulin therapy.

    Bentiromide test. Bentiromide is a synthetic peptide linked to a p-aminobenzoic acid (PABA) molecule. The patient must fast overnight and remain fasting until the test is begun. A control urine specimen is collected just before starting the test. A 500-mg test dose of bentiromide is given orally with sufficient food to stimulate the pancreas. After the bentiromide passes into the duodenum, the pancreatic enzyme chymotrypsin splits off the PABA molecule. The PABA is then absorbed into the bloodstream, conjugated in the liver, and excreted by the kidneys as arylamines. The arylamines are assayed in a 6-hour urine collection beginning with the oral dose. Decreased excretion (<50% of the test dose) suggests decreased absorption from the duodenum, which, in turn, suggests deficient activity of pancreatic chymotrypsin due to decreased pancreatic function. Sensitivity of the test for chronic pancreatitis depends to some extent on the severity of the disease, with greater sensitivity correlating with greater severity. Overall sensitivity appears to be 75%-80% (range, 39%-100%), with specificity about 90% (range, 72%-100%).

    One report obtained better results in children using a larger bentiromide dose (30 mg/kg) plus a liquid meal, and measurement of plasma PABA at 2 and 3 hours after bentiromide ingestion.

    Various conditions such as severe liver disease (interfering with conjugation), poor renal function (impaired excretion), malabsorption, incomplete urine collection, diabetes mellitus, previous gastrectomy, and inflammatory bowel disease may produce falsely decreased excretion (false positive test result). Certain medications (acetaminophen, phenacetin, lidocaine, procainamide, sulfas, thiazide diuretics, sunscreens containing PABA, and pancreatic enzyme supplements) may produce false normal results. The baseline (pretest) urine specimen can be tested to exclude presence of exogenous PABA metabolites. If there is a question of differentiating pancreatic from primary small intestine malabsorption, a d-xylose test (which is not affected by pancreatic insufficiency) can be done.

    Endoscopic retrograde cholangiopancreatography. Duodenal intubation using the ERCP technique with injection of x-ray contrast medium into the common bile duct and pancreatic ducts provides useful information about biliary tract stones or pancreatic disease in 61%-81% of cases and may be the only method that can obtain a diagnosis. However, as noted previously, ERCP requires considerable expertise and does not always succeed.

    Pancreatic stimulation tests. A tube can be passed into the duodenum with direct assay of pancreatic fluid constituents collected by duodenal tube drainage before and after injection of secretin, the pancreatic stimulating hormone. This procedure used to be considered the best diagnostic test for chronic pancreatitis and still can be useful. However, with the advent of ERCP this test has lost much of its importance. The stimulation tests do not become abnormal until 75% of pancreatic exocrine function is lost.

    Other tests. As noted previously, the D-xylose test may be useful if the patient has demonstrable steatorrhea. A normal D -xylose result is usually found in pancreatic insufficiency or in cystic fibrosis. Stool examination can be performed in a patient with steatorrhea, with differentiation of neutral fat and fatty acid. Theoretically, in pancreatic insufficiency there should be a large amount of neutral fat but very little fatty acid. Unfortunately, some of the colon bacteria apparently are able to convert neutral fat to fatty acid, so some fatty acids might be present in spite of pancreatic insufficiency in some patients. Presence of undigested meat fibers also suggests abnormal pancreatic function.

  • Acute Pancreatitis

    Classic acute pancreatitis is manifested by sudden onset of severe epigastric pain (90%-100% of patients) that may radiate elsewhere, often to the back. There may be vomiting (30%-96%), fever (60%-95%), abdominal distention (70%-80%), and paralytic ileus (50%-80%). Jaundice occasionally is present (8%-30%). Hypotension or shock develops in 30%-40% of cases. In severe and classic disease, the diagnosis is frequently obvious; unfortunately, various symptoms found in acute pancreatitis regardless of severity may occur in other diseases as well. In disease of mild or moderate degree or in patients with chronic low-grade or intermittent pancreatitis, symptoms may be vague or atypical. The most common diseases that clinically are confused with acute (or sometimes chronic) pancreatitis are perforated peptic ulcer, biliary tract inflammation or stones, intestinal infarction, and intraabdominal hemorrhage. Myocardial infarct may sometimes enter the differential diagnosis since the pain occasionally radiates to the upper abdomen; in addition, the aspartate aminotransferase (AST; formerly serum glutamic oxaloacetate transaminase, SGOT) level may be elevated in more than one half of patients with acute pancreatitis.

    Acute pancreatitis is associated with alcohol abuse or biliary tract stones in 60%-90% of cases. About 50% are associated with common duct stones (range, 20%-75%); the percentage associated with alcohol is less well documented and is more variable (probably about 20%-25% in the United States; range in different populations, 5%-49%). Alcohol use is usually heavy and longstanding. A substantial number of patients with acute pancreatitis also have cirrhosis or alcoholic liver disease. Drug hypersensitivity is another factor. Alcohol is thought to be the most common cause of chronic relapsing pancreatitis.

    Nonspecific laboratory tests

    In acute pancreatitis there is some variation in laboratory findings according to the severity of the disease. Mild or moderate leukocytosis with a neutrophil shift to the left is reported in about 80% of patients, more frequently in the more severe cases. Moderate postprandial hyperglycemia or even mild fasting hyperglycemia may be present in about one third of patients (literature range, 10%-66%). There is hyperbilirubinemia (usually mild) in 10%-20% (literature range, 10%-50%), which could be due to biliary tract stones, liver disease, or edema around the ampulla of Vater. Decreased serum calcium levels may be found in 10%-30% of cases (literature range, 10%-60%). However, since total calcium measurements include both protein-bound and nonprotein-bound (ionized) calcium, since about one half of total calcium is protein-bound, since the protein-bound fraction is predominantly bound to albumin, and since hypoalbuminemia occurs in at least 10% of acute pancreatitis cases, the serum calcium level must be correlated with the serum albumin level. Of course, there could be a coexisting artifactual and actual calcium decrease. When a calcium decrease that is not artifactually caused by hypoalbuminemia occurs, the decrease often appears 3-14 days after onset of symptoms, most frequently on the fourth or fifth day. It is attributed to the liberation of pancreatic lipase into the peritoneal cavity, with resulting digestion of fat and the combination of fatty acids with calcium, which we see grossly as fat necrosis. Again, in very severe disease there may be hemorrhagic phenomena due either to release of proteolytic enzymes such as trypsin into the blood or to release of blood into the abdominal cavity from a hemorrhagic pancreas.

    Serum amylase

    In acute pancreatitis, the most commonly used laboratory test is measurement of alpha amylase. Alpha amylase actually has several components (isoenzymes), some derived from the pancreas and some from salivary glands. Clearance from serum takes about 2 hours. A significant portion is cleared via the kidney by glomerular filtration and the remainder (some data indicate >50%) by other pathways. Serum levels become abnormal 2-12 hours after onset of acute pancreatitis in many patients and within 24 hours in about 85%-90% of cases (literature range, 17%-100%. Those studies reporting less than 75% sensitivity were in a minority and were mostly published before 1975). In most patients the serum amylase level reaches a peak by 24 hours and returns to normal in 48-72 hours. If there is continuing pancreatic cell destruction, the serum amylase level will remain elevated longer in some patients but will return to reference range in others.

    Falsely normal results. In certain situations there may be falsely low or normal serum amylase levels. The administration of glucose causes a decrease in the serum amylase level, so that values obtained during intravenous fluid therapy containing glucose may be unreliable; and one should wait at least 1 hour and preferably 2 after the patient has eaten before measuring the serum amylase value. In massive hemorrhagic pancreatic necrosis there may be no serum amylase elevation at all because no functioning cells are left to produce it. Pancreatic destruction of this degree is uncommon, however. Serum lipemia produces artifactual decrease in serum amylase values using most current methodologies. Since hypertriglyceridemia occurs in about 10%-15% of patients with acute pancreatitis (literature range, 5%-38%) and since many laboratories cannot be depended on to recognize the problem or to report the appearance of the serum, the possibility of lipemia should be considered if serum amylase results do not agree with the clinical impression.

    Important causes of elevated serum amylase levels. Following is a list of important causes of elevated serum amylase levels.

    1. Primary acute pancreatitis or chronic relapsing pancreatitis: idiopathic; traumatic; and pancreatitis associated with alcohol, drug sensitivity (thiazides, furosemide, oral contraceptives, tetracyclines, valproic acid, metronidazole), viral hepatitis, and hyperparathyroidism.
    2. Hyperamylasemia associated with biliary tract disease: cholecystitis, biliary tract lithiasis, tumor, spasm of the sphincter of Oddi produced by morphine and meperidine (Demerol) or following biliary tract cannulation.
    3. Hyperamylasemia associated with nonbiliary acute intraabdominal disease: perforated or nonperforated peptic ulcer, peritonitis, intraabdominal hemorrhage, intestinal obstruction or infarct, and recent abdominal surgery.
    4. Nonpancreatic or nonalpha amylase: acute salivary gland disease, and macroamylase.
    5. Miscellaneous: renal failure, severe cardiac circulatory failure (29% of cases), diabetic ketoacidosis in the recovery phase (41%-80% of cases), pregnancy, cerebral trauma, extensive burns, and cholecystography using radiopaque contrast medium (the contrast medium effect may last up to 72 hours in some cases).

    In some instances of biliary tract disease there is probably a retrograde secondary pancreatitis, in other cases there is release of amylase into the circulation when pancreatic duct obstruction takes place, and in still others there is no convincing anatomical explanation. Likewise, in some cases of acute nonbiliary tract intraabdominal disease there is a surface chemical pancreatitis; when intestinal obstruction or infarction occurs, there may be escape of intraluminal enzyme; but in other instances no definite cause is found.

    Serum amylase levels are elevated in about 10%-15% of patients following abdominal surgery (literature range, 9%-32%). About one half of the cases have been traced to elevation of salivary amylase levels and about one half to elevation of pancreatic amylase levels.

    Several studies have reported that serum amylase levels more than 5 times the upper reference limit are much more likely to be secondary pancreatitis caused by biliary tract disease (cholecystitis or stones in the gallbladder or common bile duct) than by idiopathic or alcoholic acute pancreatitis. However, this is not sufficiently reliable by itself to differentiate primary and secondary pancreatic disease. The serum amylase level is said to be normal in most patients with pancreatic carcinoma, but occasionally some degree of acute pancreatitis may coexist with the tumor.

    Patients with poor renal function may have false elevation of serum amylase, pancreatic amylase isoenzyme, lipase, immunoreactive trypsin, and amylase/creatinine clearance ratio. In one study it was reported that a creatinine clearance value of 40 ml/min represented the degree of renal function beyond which false enzyme elevation began to occur. However, regardless of severity of renal failure, some patients had enzyme values that remained within the normal range (60% [ literature range, 40%-81%;] had elevated amylase and 60% had elevated lipase). One study found that the highest false amylase elevation was 5 times normal (4 times normal in chronic renal failure); for lipase, 6 times normal; and for trypsin, 5.5 times normal. Patients with acute renal failure have higher values than those with chronic renal failure.

    Sensitivity and specificity of alpha amylase as a test for acute pancreatitis has varied considerably in reports using different manufacturer’s kits and even between investigators using the same kit. As noted previously, the average sensitivity in acute pancreatitis seems to be about 85%-90% (range, 17%-100%), with specificity about 45%-50% (range, 0%-89%). In nonpancreatic diseases with elevated amylase levels, the frequency of elevated values above 3 times the upper reference limit was about 15% (range, 0%-36%).

    Urine amylase

    Urine amylase determination may also be helpful, especially when the serum amylase level is normal or equivocally elevated. Urine amylase usually rises within 24 hours after serum amylase and as a rule remains abnormal for 7-10 days after the serum concentration returns to normal. Various investigators have used 1-, 2-, and 24-hour collection periods with roughly equal success. The shorter collections must be very accurately timed, whereas the 24-hour specimen may involve problems in complete collection. It is important to have the results reported in units per hour. Frequently the values are reported in units/100 ml, but such values are inaccurate because they are influenced by fluctuating urine volumes. One drawback to both serum and urine amylase determination is their relation to renal function. When renal function is sufficiently diminished to produce serum blood urea nitrogen elevation, amylase excretion also diminishes, leading to mild or moderate elevation in serum amylase levels and a decrease in urine amylase levels.

    Amylase/creatinine clearance ratio

    Because renal excretion of amylase depends on adequate renal function, amylase urinary excretion correlates with creatinine clearance. In acute pancreatitis, however, there seems to be increased clearance of amylase compared with creatinine. The amylase/creatinine clearance ratio (A/CCR) is based on this observation. Determination of A/CCR involves “simultaneous” collection of one serum and one urine specimen and does not require a timed or complete urine collection. The A/CCR becomes abnormal 1-2 days after elevation of serum amylase levels but is said to remain abnormal about as long as urine amylase. The A/CCR has been the subject of widely discrepant reports. Early investigators found more than 90% sensitivity for acute pancreatitis. Later reports indicated a sensitivity varying from 33%-75%. One great problem in evaluating reports of sensitivity for any biochemical pancreatic function test is the fact that there is no noninvasive perfect way to detect all cases of acute pancreatitis (while at the same time not producing false abnormality due to nonpancreatic disease) against which the various tests may be compared, and even the invasive diagnostic procedures may fail to detect relatively mild disease.

    The A/CCR is more specific for pancreatitis than changes in the serum amylase level. Many of the etiologies for hyperamylasemia that do not evoke a secondary pancreatitis are associated with normal A/CCR values. The exact degree of specificity is not yet established; reports have appeared that A/CCR may be elevated in some cases of diabetic ketoacidosis and burns. Behavior in renal failure is variable. In mild azotemia, the A/CCR may be normal, but in more severe azotemia or in uremia sufficient to require dialysis, it may be elevated. In addition, different investigators have adapted different ratio numbers as upper limits of normal, and there have been suggestions that the particular amylase method used can influence results. Some investigators feel that the A/CCR ratio has little value. More data is needed before final conclusions about A/CCR can be made.

    Notwithstanding the limitations of the A/CCR previously noted and the ongoing debate in the literature regarding its usefulness, the sensitivity and specificity results for other tests suggest that the A/CCR is probably the most reliable of the readily available tests for acute pancreatitis. However, since there are serious questions about its sensitivity, serum and urine amylase measurements would be helpful if the A/CCR result is within reference limits. Therefore, if the urine amylase specimen is collected as a timed specimen, the result of urine amylase test as well as that of the serum amylase test would be available as a single test result. Urine amylase is especially useful since it remains elevated longer than serum amylase and is not affected by macroamylasemia. Poor renal function can be a disruptive factor in all of these tests.

    Macroamylase is a macromolecular complex that contains alpha amylase bound to other molecules. Although macroamylase is thought to be uncommon, two studies detected it in 1.1%-2.7% of patients with elevated serum amylase levels. Macroamylase does not pass the glomerular filter but accumulates in serum; if a serum amylase test is performed, the macroamylase will be included in the amylase measurement and may produce an elevated test result. This could simulate pancreatic disease. Since macroamylase does not reach the urine, the urine amylase level is normal or low. The combination of an elevated serum amylase level and a normal or low urine amylase level produces a low A/CCR, and the elevated serum amylase level plus reduced A/CCR has been used to diagnose macroamylasemia. However, in early acute pancreatitis the serum amylase level may be elevated before the urine amylase level becomes elevated. Also, since occasionally patients with elevated serum amylase levels due to salivary (rather than pancreatic) amylase may have a reduced A/CCR, macroamylase should be confirmed by special techniques such as selective chromatography. Renal failure introduces an additional source of confusion, since both serum amylase and lipase levels are frequently elevated and the A/CCR may not be reliable. Other hints that an elevated serum amylase level might be due to macroamylase would include a normal serum lipase level and failure of the elevated serum amylase level to decrease significantly over several additional days.

    Amylase isoenzyme fractionation

    Alpha amylase consists of two groups of isoenzymes: pancreatic and salivary. Each group consists of more than one isoenzyme. Separation of serum amylase into its component isoenzymes is possible by selective enzymatic or chemical inhibition or by electrophoresis. In clinical acute pancreatitis, reports indicate that the expected increase in pancreatic-type isoenzymes is observed. In hyperamylasemia without clinical pancreatitis (e.g., occurring during diabetic ketoacidosis or after abdominal surgery), some patients exhibit increased salivary type isoenzymes and others, increased pancreatic type. Pancreatic isoenzyme kits have only recently become commercially available. The majority use an enzyme derived from wheat germ that inhibits salivary isoenzyme. Most evaluations to date found that isoenzyme fractionation was very helpful, especially since the finding of elevated salivary-type isoamylase without the pancreatic type would suggest a nonpancreatic amylase source. Although theoretically the pancreatic-type isoenzyme should be specific for pancreatic origin, a minority of investigators reported relatively frequent elevation of pancreatic-type isoenzyme in several nonpancreatic conditions.

    Serum lipase

    The serum lipase level is considered more specific for pancreatic damage than the amylase level. Lipase levels rise slightly later than the serum amylase levels, beginning in 3-6 hours, with a peak most often at 24 hours, and tend to remain abnormal longer, in most instances returning to reference range in 7-10 days. Lipase is excreted by filtration through renal glomeruli, after which most is reabsorbed by the renal proximal tubules and catabolized elsewhere. Urine lipase assay is not currently used. Evaluation of serum lipase sensitivity and specificity for acute pancreatitis has shown considerable variation, with sensitivity averaging about 75%-80% (range, 18%-100%) and with specificity averaging about 70% (range, 40%-99%). Some consider lipase very sensitive and specific for acute pancreatic disease, especially methods using a lipase cofactor called colipase. In general, however, the consensus in the literature is that lipase is probably about 10% less sensitive than serum amylase but is about 20%-30% more specific. In those reports that indicated less specificity, lipase elevations were found in some (but not as many) of the same nonpancreatic conditions associated with elevated serum amylase. Renal failure is the nonpancreatic condition most frequently associated with elevated serum lipase levels. About 80% of patients with renal failure are said to have lipase elevation 2-3 times the upper limit of reference range; about 5% have elevation over 5 times the upper limit. Other conditions that sometimes elevate serum lipase levels are acute cholangitis, intestinal infarction, and small intestine obstruction. In most patients with these conditions, lipase elevations are less than 3 times the upper limit. Other associated conditions include mumps, extrahepatic biliary obstruction, acute cholecystitis, peptic ulcer, and pancreatic carcinoma. Some of these conditions, however, could actually be associated with acute pancreatitis. Lipemia produces falsely decreased serum lipase and serum amylase levels.

    Serum immunoreactive trypsin

    Several investigators in recent years have developed radioimmunoassay (RIA) procedures for se rum trypsin, and at least one manufacturer has a commercial kit available. Trypsin is produced exclusively by the pancreas. In serum a considerable proportion is bound to alpha-1 antitrypsin, and some is also complexed to alpha-2 macroglobulin. Normally the trypsin activity in serum (as measured by current RIA techniques) actually is not trypsin but the trypsin precursor trypsinogen. In acute pancreatic disease there is activation of trypsinogen to form trypsin. Some of the RIA techniques described in the literature measure trypsin bound to alpha-1 antitrypsin as well as trypsinogen, and some do not. None measure trypsin complexed to alpha-2 macroglobulin.

    Serum immunoreactive trypsin (SIT) levels are reported to be elevated in 95%-100% of patients with acute pancreatitis or acute exacerbation of chronic pancreatitis. They are also elevated in 80%-100% of patients with renal failure. There are insufficient and conflicting data on SIT behavior in nonpancreatic disorders, with some investigators reporting normal results in patients with cirrhosis and biliary tract disease and others reporting elevation in more than one half of patients with common duct stones and 6%-16% of patients with cirrhosis (the larger incidence being alcoholic cirrhosis). One investigator found elevated values in patients with viral infections such as mumps. In one report 50% of patients with pancreatic carcinoma had elevated values, whereas 19% had subnormal values.

    Because only large medical centers or reference laboratories currently offer the test, the time delay necessary for results makes SIT less useful for diagnosing acute pancreatitis.

    Other tests. Carboxypeptidase A and phospholipase A have been advocated to diagnose acute pancreatitis. Both have had mixed evaluation reports and at present do not appear to have clear-cut advantages over current tests. The carboxypeptidase A level does remain elevated longer than serum amylase or lipase levels and is a little more specific for pancreatic disease.

    Endoscopic retrograde cholangiopancreatography

    As the term implies, endoscopic retrograde cholangiopancreatography (ERCP) entails placing a special side-viewing fiberoptic duodenoscope into the duodenum under fluoroscopic control, finding the ampulla of Vater, cannulating the ampulla, and injecting x-ray contrast media into either the common bile duct or the pancreatic duct, or both. In the case of the pancreas, alterations in pancreatic duct x-ray morphology can suggest acute pancreatitis, chronic pancreatitis, pancreatic carcinoma, and pancreatic cyst. Clear separation of all of these entities from each other is not always possible in every patient. Current consensus is that ERCP is the most sensitive and reliable single procedure to detect clinically significant degrees of pancreatic dysfunction and to establish normal function (this does not take into account minimal or minor degrees of pancreatic disease, since the test depends on alteration of duct structure from abnormality in surrounding parenchyma). It is said to be especially useful when pancreatic ductal surgery or drainage procedures are being considered, in patients with possible traumatic injury to the pancreas, in cases where other diagnostic modalities fail to yield a clear-cut diagnosis, and in cases where strong clinical suspicion of pancreatic disease exists but other modalities are normal.

    Disadvantages of ERCP are insensitivity to mild or minimal pancreatic disease as noted previously, the invasiveness of the procedure, the need for very experienced endoscopists and radiologists, and the considerable cost involved. About 10%-20% of attempts fail for various technical reasons. Complications of ERCP are uncommon (<2%), with sepsis and self-limited episodes of acute pancreatitis being the most frequent problems. There is a transient increase in serum amylase levels after ERCP in about 40% of patients. If there is barium in the GI tract that could obscure x-ray visualization of the pancreatic duct system, ERCP of the pancreas cannot be performed.

    Computerized tomography and ultrasound

    Both computerized tomography (CT) and ultrasound can visualize the pancreas, and both have their enthusiasts. Ultrasound is a little better in very thin persons, and CT is better in obese persons. Both can detect abnormality of the pancreas in about 80%-85% of patients. Neither ultrasound nor CT can always delineate the normal pancreas, although failure is less frequent with newer CT models. In general, in the average hospital CT of the pancreas is easier to perform and interpret than ultrasound. However, ultrasound is generally considered the best procedure for the diagnosis of pancreatic pseudocyst. Both CT and ultrasound depend on focal or generalized gland enlargement, so that small tumors or mild generalized disease are likely to be missed.

  • Diarrhea

    Differential tests

    There are many conditions that produce a chronic diarrhea, which must be differentiated from the relatively common types that last only a few days and usually respond to ordinary treatment. Diarrhea in infants will not be specifically discussed, since this is a special problem peculiar to that age group.

    In patients of all ages with long-term or chronic diarrhea, a stool should be obtained for culture to rule out the presence of Salmonella or Shigella bacteria. In some areas, Campylobacter or Yersinia infection might be the etiology. A stool should also be obtained for ova and parasites, with special emphasis on the possibility of amebae or Giardia being present. In children, malabsorption is caused either by cystic fibrosis of the pancreas or celiac disease. In adults, the various forms of sprue and, more rarely, some of the secondary malabsorption causes might be considered. In children and young and middle-aged adults, ulcerative colitis is a possibility, especially if there is blood in the stools. This calls for a sigmoidoscopic or colonoscopic examination. In adults over age 40 years, carcinoma of the colon is the cause of diarrhea in a significant number of cases. A barium enema and colonoscopic examination are necessary. In the aged, in addition to carcinoma, fecal impaction is a frequent cause of diarrhea, and this usually can be determined easily by ordinary rectal examination.

    One study reports that diarrhea is more frequent in patients with serum albumin levels less than 2.5 g/100 ml (25 g/L). Chronic diarrhea in diabetics occurs in 8%-22% of patients; in one study, about 50% had a known cause determined. It is more frequent in patients with poorly controlled diabetes treated with insulin who also have peripheral neuropathy and autonomic nervous system dysfunction. The diarrhea may be intermittent. Steatorrhea may or may not be associated. In many cases no organic etiology for persistent diarrhea can be found. This situation is often called “functional diarrhea” and is attributed to psychiatric causes. The organic diseases listed here must be ruled out before deciding that a patient has a psychosomatic disorder.

    Differential Diagnosis of Diarrhea (More Frequent Etiologies)

    Infection—bacterial
    Salmonella, Shigella, Campylobacter, Yersinia enterocolitica, enteropathic Escherichia coli, Clostridium difficile enterocolitis.
    Infection—virus
    Rotovirus, fastidious enteric adenovirus, Norwalk virus
    Infection—parasites
    Giardia lamblia, Entamoeba histolytica
    Ulcerative colitis—regional enteritis
    Partial obstruction of colon
    Colon carcinoma
    Fecal impaction
    Malabsorption—steatorrhea
    Celiac disease: nontropical sprue, tropical sprue, disaccharide enzyme deficiency
    Other
    Diabetic neuropathy
    Zollinger-Ellison syndrome
    Hypoalbuminemia-associated

    Persons infected by the human immunodeficiency virus 1 (HIV-1) often develop diarrhea, especially if they progress to the stages of disease known as acquired immunodeficiency syndrome (AIDS) or AIDS-related complex. Common infecting organisms in these patients are Mycobacterium avium, Mycobacterium intracellulare, Salmonella, Cryptosporidium, Microsporidium, cytomegalovirus, Giardia, Strongyloides stercoralis, and Isospora belli. However, numerous other organisms have been reported.

  • Tests for Gastric Blood

    Until the middle of the 1980s, there was no commonly accepted method to test for blood in gastric contents. Methods I have personally seen used in different laboratories include urine dipsticks, orthotolidine tablets, and guaiac-impregnated filter paper. Studies have indicated that small numbers of red blood cells (RBCs) are present in most gastric aspirates without clinical evidence of bleeding. These RBCs are often sufficient to produce a reaction with orthotolidine or the urine dipsticks. The guaiac-impregnated filter paper tests used for stool occult blood are less sensitive and also appear to detect clinically significant amounts of blood but were shown to lose sensitivity at pH levels below 3.0 (literature range, 2.5-4.0). In addition, cimetadine, a medication frequently used to decrease gastric acid production, can produce false negative guaiac tests. A new guaiac test introduced in 1985 called Gastroccult is buffered so as to maintain sensitivity down to pH 1.0. In addition, the reagent contains a substance that inhibits plant peroxidase and thereby decreases chances of a false positive result from that source. Also, Gastroccult is not affected by cimetadine. Gastroccult has a sensitivity of approximately 5 mg of hemoglobin/100 ml of gastric contents (equivalent to about 50 µl of whole blood/100 ml of gastric contents), with a detection range in the literature of 30-200 µl of blood/100 ml of diluent. Although there is no consensus regarding the exact amount of gastric blood considered significant, 50 µl/100 ml seems to be acceptable. Ascorbic acid (vitamin C) inhibits both the standard guaiac test and Gastroccult. Large amounts of antacids may also inhibit the test reaction; the manufacturer states that this possibility must be taken into consideration if testing is done within 1 hour after administering the antacid.

  • Gastric Analysis

    Gastric analysis has two main uses: to determine gastric acidity and to obtain material for exfoliative cytology. I shall discuss only the first here.

    When gastric aspiration is performed to determine gastric acidity, the usual purpose is either (1) to determine the degree of acid production in persons with ulcer or ulcer symptoms of (2) to determine if the stomach is capable of producing acid as part of a workup for pernicious anemia. Since passing the necessary large-caliber tube is not met with enthusiasm by the patient, it is important that the physician understand what information can be obtained and be certain that this information is really necessary.

    Outdated gastric acidity method. One problem in evaluating gastric acid secretion data from the literature is the term “achlorhydria,” which is often used as a synonym for “anacidity.” The classic method of gastric analysis involved titration with 0.1N sodium hydroxide to the end point of Topfer’s reagent (pH, 2.9-4.0); this represented “free HCl.” Next the specimen was titrated to the end point of phenolphthalein (pH, 8.3-10.0); this represented “total acid.” The difference was said to represent “combined acid,” thought to consist of protein-bound and weak organic acids but probably including small amounts of HCl. Achlorhydria technically is defined as absence of free acid (pH will not drop below 3.5 on stimulation) but not necessarily complete lack of all acid. True anacidity is absence of all acid, now defined as a pH that does not fall below 6.0 or decrease more than 1 pH unit after maximum stimulation. Therefore, achlorhydria is not the same as anacidity. Nevertheless, the two terms are often used interchangeably. Gastric acidity by the old method was reported in degrees or units; this was the same as milliequivalents per liter. Reference values for total 12-hour gastric secretion were 20-100 ml and for 12-hour total acid content were 10-50 mEq/L (literature range, 2-100 mEq/L).

    Currently recommended gastric acidity procedure. All authorities today recommend that the old gastric acidity procedure be replaced by a timed collection protocol with results reported in milliequivalents per hour, that is, secretion rate instead of concentration. A 1-hour basal specimen is collected (basal acid output [BAO]). Reference values are not uniform but seem most often to be quoted as 1-6 mEq/hour. An acid production stimulant is then injected. Either pentagastrin, betazole (Histolog), or histamine can be used; pentagastrin has the fewest side effects and histamine the most. After injection of the acid stimulant, four 15-minute consecutive specimens are collected (using continuous suction if possible). Maximum acid output (MAO) is the sum of all four 15-minute poststimulation acid collections. Acidity can be measured by titration with the chemical indicator methyl red, but many laboratories now use a pH electrode.

    Proper placement of the gastric tube is critical; many recommend assistance by fluoroscopy. Reference values for MAO are less than 40 mEq/hour. The BAO/MAO ratio should be less than 0.3.

    Conditions in which gastric analysis is useful

    Diagnosis of pernicious anemia. Presence of acid secretion rules out pernicious anemia. Complete lack of acid secretion after maximum stimulation is consistent with pernicious anemia but may occur in up to 30% of persons over age 70 and occasionally in presumably normal younger persons. If basal secretion fails to demonstrate acid, stimulation is necessary. Alcohol or caffeine stimulation has been used; but since these agents do not produce maximal stimulation of acid production, it would be necessary to repeat the test using pentagastrin, betazole, or histamine if no acid production were found. Therefore, a stronger stimulant, such as pentagastrin, is preferred as the original stimulation agent. Anacidity rather than achlorhydria is the classic gastric analysis finding in pernicious anemia; but, as noted previously, the older term achlorhydria is still being used with the same meaning as anacidity.

    Many hematologists perform the Schilling test without preliminary gastric analysis if they suspect pernicious anemia. If the Schilling test produces clear-cut evidence either for or against pernicious anemia, gastric aspiration usually is not necessary. This is especially true if test results are definitely normal, since the greatest technical problem of the Schilling test is incomplete urine collection leading to a falsely low result. If the Schilling test result is equivocal, or if there is some doubt regarding an excretion value suggesting pernicious anemia, gastric aspiration can still be carried out since it is not affected by the Schilling test.

    Diagnosis of gastric cancer. Given a known gastric lesion, anacidity after maximum stimulation is strong evidence against peptic ulcer. However, only about 20% of gastric carcinomas are associated with complete anacidity, so gastric analysis in most cases has been replaced by fiberoptic gastroscopy with direct visualization and biopsy of the lesion.

    Diagnosis of Zollinger-Ellison syndrome.

    These patients have a gastrin-producing tumor, usually in the pancreas, and typically demonstrate a high basal acid secretion with minimal change after stimulation. Specifically, gastric analysis is strongly suggestive when the BAO is 15 mEq/hour or the BAO/MAO ratio is 0.6 or greater (i.e., BAO is 60% or more of the MAO after maximum stimulation). Some consider a BAO of 10 mEq/hour and a BAO/MAO ratio greater than 0.4 as evidence suggesting a need for further workup so as not to miss a gastrin-producing tumor. About 70% of Zollinger-Ellison patients have a BAO more than 15 mEq/hour (literature range, 50%-82%) as opposed to about 8% of duodenal ulcer patients (literature range, 2%-10%). About 55% of patients with Zollinger-Ellison syndrome (literature range, 35%-75%) have a BAO/MAO ratio higher than 0.6 as opposed to about 2% (literature range, 1%- 5%) of duodenal ulcer patients. The definitive diagnostic procedure for gastrinoma is serum gastrin assay. If Zollinger-Ellison syndrome is a possibility, many physicians proceed directly to serum gastrin assay without gastric acid studies.

    Diagnosis of marginal ulcer. After partial gastric resection with gastrojejunostomy (Billroth II procedure or one of its variants), abdominal pain or GI bleeding may raise the question of ulcer in the jejunum near the anastomosis. An MAO value above 25 mEq/hour is strongly suggestive of marginal ulcer; MAO less than 15 mEq/hour is evidence against this diagnosis.

    Differentiation of gastric from duodenal ulcer. Duodenal ulcer patients as a group tend to have gastric acid hypersecretion, whereas gastric ulcer patients most often have normal or even low rates. Patients with gastric ulcer usually have MAO values less than 40 mEq/hour. About 25%-50% of duodenal ulcer patients have MAOs greater than 40 mEq/hour. Conversely, very low acid secretion rates are evidence against duodenal ulcer. Basal secretion greater than 10 mEq/hour is evidence against gastric ulcer.

    Determining type and extent of gastric resection. Knowing the amount of acid is sometimes helpful in the surgical treatment of duodenal ulcer. Some surgeons prefer to do a hemigastrectomy (removal of one half of the stomach) rather than a subtotal gastrectomy (two-thirds resection) because postoperative complications are fewer with a hemigastrectomy. If the patient is a hypersecretor, the surgeon may add vagotomy to a hemigastrectomy or may perform a subtotal resection to reduce HCl-producing cells or lessen stimulation of those that remain.

    Evaluation of vagotomy status. Patients undergoing a surgical procedure that includes bilateral vagotomy may later experience symptoms that might be due to recurrent ulcer or manifest a proved recurrent ulcer. The question then arises whether vagotomy is complete. The Hollander test employs insulin hypoglycemia (20 units of regular insulin, or 0.1 unit/kg) to stimulate gastric acid secretion through intact vagal nerve fibers. Although disagreement exists on what values are considered normal, most physicians use (1) a BAO less than 2mEq/hour; and (2) for postinsulin values, either total acid output less than 2 mEq/hour in any 1-hour period or an increase in acid concentration of less than 20 mEq/hour in 2 hours. Most agree that a “positive” response means incomplete vagal section. Interpretation of the “negative” response (failure to secrete sufficient acid under stimulus of hypoglycemia) is more controversial. Antrectomy or partial gastrectomy removes HCl-secreting cells, and a negative response thus could be due either to vagal section or to intact vagus but insufficient total gastric HCl secretory activity.