Cystic fibrosis. Cystic fibrosis (mucoviscidosis, or fibrocystic disease of the pancreas) is the most common eventually lethal autosomal recessive inherited disorder in Europeans (estimated gene frequency of 1 in 2,000 live births). Incidence in African Americans is 2% of that in Europeans; it is rare in Asians. About 90% of homozygotes have symptoms resulting predominately from damage to mucus-producing exocrine glands, although non-mucus-producing exocrine glands can also be affected. The mucus glands produce abnormally viscid secretions that may inspissate, plug the gland ducts, and generate obstructive complications. In the lungs, this may lead to recurrent bronchopneumonia, the most frequent and most dangerous complication of cystic fibrosis. Pseudomonas aeruginosa and Staphylococcus aureus are the most frequent pathogens. The next most common abnormality is complete or partial destruction of the exocrine portions of the pancreas, leading to various degrees of malabsorption, steatorrhea, digestive disturbances, and malnutrition. This manifestation varies in severity, and about 15% of patients have only a minimal disorder or even a normal pancreatic exocrine function. Less common findings are biliary cirrhosis, most often focal, due to obstruction of bile ductules; and intestinal obstruction by inspissated meconium (meconium ileus), found in 10%-15% of newborns with cystic fibrosis. Gamma-glutamyltransferase (GGT) enzyme is elevated in about one third of patients, predominantly those with some degree of active bile duct injury or cirrhosis.

Sweat test for screening. Non-mucus-producing exocrine glands such as the sweat glands do not ordinarily cause symptoms. However, they are also affected, because the sodium and chloride concentration in sweat is higher than normal in patients with cystic fibrosis, even though the volume of sweat is not abnormally increased. Therefore, unusually high quantities of sodium and chloride are lost in sweat, and this fact is utilized for diagnosis. Screening tests (silver nitrate or Schwachman test) that depend on the incorporation of silver nitrate into agar plates or special paper have been devised. The patient’s hand is carefully washed and dried, since previously dried sweat will leave a concentrated chloride residue on the skin and give a false positive result. After an extended period or after exercise to increase secretions, the palm or fingers are placed on the silver nitrate surface. Excess chlorides will combine with the silver nitrate to form visible silver chloride. However, this method is not accurate in patients less than 2 months old.

Sweat test for diagnosis. For definitive diagnosis, sweat is collected by plastic bag or by a technique known as iontophoresis. Iontophoresis using the Gibson-Cooke method is the current standard procedure. Sweat is induced by sweat stimulants such as pilocarpine. The iontophoresis apparatus consists of two small electrodes that create a tiny electric current to transport the stimulating drug into the sweat glands of the skin. The sweat is collected in small gauze pads. The procedure is painless. According to a report from a committee sponsored by the Cystic Fibrosis Foundation in 1983, the standard Gibson-Cooke method is difficult to perform in neonates, and it is better to wait until age 4 weeks if possible. Modifications of the equipment and collection system have been devised and are commercially available.

In children, a sweat chloride content greater than 60 mEq/L (60 mmol/L) or a sweat sodium content greater than 70 mEq/L (70 mmol/L) is considered definitely abnormal. Sodium and chloride may normally be higher (75-80 mEq/L) during the first 3 days of life, decreasing to childhood values by the fourth day. In children there is an equivocal zone (50-80 mEq/L chloride, possibly even up to 90 mEq/L) in which the diagnosis should be considered unproved. Repeated determinations using good technique and acquiring an adequate quantity of sweat are needed. Volumes of sweat weighing less than 50 mg are not considered reliable for analysis. For diagnostic sweat collection it is recommended that the hand not be used, because the concentration of electrolytes in the palm is significantly greater than elsewhere. A further caution pertains to reference values in persons over age 15 years. Whereas in one study there were only 5% of children with cystic fibrosis who had sweat chloride values less than 50 mEq/L and 3% of controls with values of 60-70 mEq/L, 34% of a group of normal adults were found to have sweat sodium concentration greater than 60 mEq/L and in 4% values were more than 90 mEq/L. Another report did not verify these data; therefore, the diagnosis may be more difficult in adults.

One might gather from this discussion that sweat electrolyte analysis is no more difficult than standard laboratory tests such as blood gas analysis or serum protein electrophoresis. Unfortunately, surveys have shown that the majority of laboratories have a relatively low level of accuracy for sweat testing. The newer commercially available iontophoresis modifications have not shown that they can consistently achieve the accuracy of a carefully performed Gibson-Cooke analysis, although some evaluations have been very favorable. Authorities in cystic fibrosis strongly recommend that the diagnosis of cystic fibrosis should not be made or excluded with certainty on the basis of a single sweat analysis. A minimum of two tests showing unequivocal results with adequate control results are necessary. It is preferable to refer a patient with symptoms suggestive of cystic fibrosis to a specialized cystic fibrosis center experienced in the Gibson-Cooke technique to obtain a definitive diagnosis.

Clinically normal heterozygotes and relatives of patients with cystic fibrosis have been reported to have abnormal sweat electrolytes in 5%-20% of instances, although some investigators dispute these findings.

Some investigators feel that sweat chloride provides better separation of normal persons from persons with cystic fibrosis than sweat sodium.

DNA linkage analysis. The gene causing cystic fibrosis is located on the long arm of chromosome 7 (7q). The most common variant of cystic fibrosis (70% of cases) results from deletion of the 3-nucleotide sequence of a phenylalanine molecule in amino acid position (codon) 508 (often called df508) of the cystic fibrosis gene. When combined with the four next most common gene abnormalities, current DNA probe techniques using the indirect linkage analysis method reportedly have a sensitivity of 85%. Some are using probes for more than five genetic defects and claim a sensitivity of 90% or more. This technique can be applied to prenatal diagnosis in the first trimester using a chorionic villus biopsy specimen.

Other screening tests. Other tests have been suggested to screen for cystic fibrosis. Trypsin is an enzyme produced only in the pancreas. Serum trypsin is elevated in the first months or years of the disease (as the pancreatic cells are destroyed) but eventually decreases below the reference range. However, this leads to considerable overlap with normal persons (in the transition stage between elevated values and decreased values), so that only unequivocally high or low values are significant. The precursor of trypsin is trypsinogen, and levels of this proenzyme are elevated in most neonates with cystic fibrosis and can be measured by immunoassay (immunoreactive trypsinogen, IRT). IRT assay can be performed on filter paper dried blood spots in a similar manner to other neonatal congenital disease screening. Several screening projects have been reported with good results. However, the Cystic Fibrosis Committee noted that various immunoassays have not been standardized, cutoff detection levels are not uniform, and there is a possibility that some of the 10% of infants with cystic fibrosis who have normal pancreatic function could be missed. IRT usually declines in infants with cystic fibrosis after a few weeks, so that repeat testing results would be difficult to interpret. Also, IRT may be normal in some infants with meconium ileus. A 1991 state screening program using IRT detected 95% of infants with cystic fibrosis and used a lower cutoff point on repeat testing to compensate for expected decrease in IRT.

Other methods involve testing meconium in the first stools produced by newborns. One technique tests for increased protein levels (which are predominantly albumin) using a paper dipstick that detects elevated levels of albumin. Since 15%-25% of infants with cystic fibrosis have normal degrees of pancreatic enzyme activity, the test yields at least that number of false negative results. In addition, it yields a considerable number of false positive results. The greatest number of false positives (about 50% of all positive specimens in one study) comes from low-birth-weight infants. Other causes for false positive results include contamination by blood, protein from infant formula, and protein in baby cream. Another approach involves a test for glucose on the meconium stools, which is supposed to reflect the presence or absence of lactase activity. In one study, about one third of cystic fibrosis cases were missed by both the albumin and the glucose (lactose activity) tests.