Sweat test

The Sweat test measures the concentration of chloride and sodium that is excreted in sweat. It is used to diagnose cystic fibrosis (CF).

Additional recommended knowledge

Weighing the right way

Essential Laboratory Skills Guide

Daily Visual Balance Check

Method

Sweating is induced by pilocarpine iontophoresis. At the test site, an electrode is placed over gauze containing pilocarpine and electrolyte solution that will not interfere with the sodium and chloride measurement. A second electrode (without pilocarpine) will be placed at another site and a mild electrical current will draw the pilocarpine into the skin where it stimulates the sweat glands.

The test site is carefully cleaned and dried, then a piece of preweighed filter paper is placed over the test site and covered with paraffin to prevent evaporation. Specialized collection devices may also be used. Sweat is collected for 30 minutes. The filter paper is retrieved and weighed to determine the weight of sweat collected. Several laboratory methods are then used to determine the sodium and chloride concentrations.

Results

Reference ranges

If the concentration of chloride is >60 mEq/L, the test is positive; 40-60 mEq/L is borderline; <40 mEq/L is negative. The reference range for sodium is <70-90 mEq/L. The minimum sample weight varies with the collection method.

Interpretation

Two reliable positive results on two separate days is diagnostic for CF. Because of the existence of milder variants, borderline or even near-borderline negative results may be used to diagnose CF. Clinical presentation, family history and patient age must be considered to interpret the results. Highly discordant sodium and chloride values may indicate technical errors.

Sources of Error

Technical errors, insufficient sample, evaporation, contamination, dehydration, mineralocorticoid hormone therapy, and skin rash on the tested area may produce incorrect results. False positive test results may be caused by malnutrition, adrenal insufficiency, glycogen storage diseases, hypothyroidism, hypoparathyroidism, nephrogenic diabetes insipidus, G6PD deficiency or ectodermal dysplasia (source: http://www.eMRCP.com)