Sweat testing methods for the diagnosis of cystic fibrosis - Tips from Other Journals

Author: Richard Sadovsky
Date: May 1, 1997

Although the genetic link to cystic fibrosis has been clearly defined, the mainstay of diagnosis remains the sweat test. Confirmation of diagnosis by genetic testing is limited because of the large numbers of gene mutations. The evaluation of sweat testing depends on the type of test performed, the skill of the technician and the physician's ability to analyze the laboratory data. LeGrys employs a question and answer format to discuss the use and proper evaluation of sweat test results.

The sweat test generally has three parts: sweat stimulation by pilocarpine iontophoresis; collection of the sweat onto gauze, filter paper, coil, patch or capillary tube, and quantitative or qualitative analysis of the sweat for the chloride concentration, the sodium concentration, conductivity or osmolality. A qualitative sweat test is a screening test for cystic fibrosis. Positive or borderline results should be followed by a quantitative test measuring the amount of sweat and the chloride and sodium concentrations.

According to the United States Cystic Fibrosis Foundation, a quantitative analysis of sweat chloride, with or without sodium, using reliable methods is acceptable. The most commonly accepted test method is the quantitative pilocarpine iontophoresis test (QPIT). At least two positive QPIT results along with a suggestive clinical presentation are diagnostic of cystic fibrosis. A sweat chloride concentration from 40 to 60 mEq per L (40 to 60 mmol per L) is considered borderline, and a concentration greater than 60 mEq per L (60 mmol per L) is consistent with cystic fibrosis. Patients should be at least 48 hours old before undergoing a sweat test. Sweat electrolyte concentration increases with age, and adults can have sweat chloride concentrations greater than 60 mEq per L (60 mmol per L). Sweat conductivity is a qualitative test and can be used for screening.

False sweat test results can occur because of methodologic errors, technical errors and errors in interpretation. Sample evaporation and condensation must be avoided. Standard procedures have been published by the National Committee for Clinical Laboratory Standards and need to be followed. Physiologically, the maximum sweat test is around 160 mEq per L (160 mmol per L). Results significantly higher than this should prompt one to consider a review of technique or evaluation for Munchausen's syndrome by proxy.

Some patients fail to produce an adequate amount of sweat because of skin conditions or factors such as age, race, or collection system. Sweat collection cannot be extended for more than 30 minutes because of the risk of evaporation. Many laboratories perform the sweat test in duplicate, using collections from two different sites for quality assurance.

Disorders that can cause an elevated sweat electrolyte concentration include anorexia nervosa, atopic dermatitis, autonomic dysfunction, nephrosis, protein calorie malnutrition, psychosocial failure to thrive, untreated hypothyroidism or adrenal insufficiency, and others. Most of these disorders are usually distinguishable from cystic fibrosis on the basis of clinical presentation.

Complications of sweat testing are minimal and include the slight potential of a small skin burn if the iontophoresis current is excessive or if the electrode surface is defective. Local urticaria is rare. The laboratory report for a sweat test should include information on the type of test used, the analyte of sweat being measured, the volume of sweat and the results obtained. The interpretation depends on the physician's knowledge of the analytical method and the laboratory reliability.

The author concludes that sweat testing remains the most reliable screening and diagnostic tool for cystic fibrosis. However, test results must be considered along with laboratory reliability and clinical presentation.

RICHARD SADOVSKY, M.D. LeGrys VA. Sweat testing for the diagnosis of cystic fibrosis: practical considerations. J Pediatr 1996;129:892-7.

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