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Outdated gastric acidity method.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.