Test, sweat chloride

A common and simple test used to evaluate a patient who is suspected of having cystic fibrosis (CF), the most common lethal genetic disease affecting Caucasians. The goal of the test is to painlessly stimulate the patientąs skin to produce a large enough amount of sweat which may then be absorbed by a special filter paper and analyzed for the content of chloride in the sweat. To produce a good volume of sweat requires application of a minute (painless) electric current which allows penetration of a medication which maximizes sweat stimulation. The technique is called iontophoresis. The patientąs forearm is commonly used; however, in small infants, the back may also serve as an appropriate area to perform this procedure. The test usually takes about a half hour to an hour. The sweat is collected on a specialized filter paper. After determining that enough sweat has been collected to ensure test reliability, the amount of chloride in the sweat is measured. The normal sweat chloride values are 10-35 milliequivalents per liter. Patients with CF usually have a sweat chloride value greater than 60 milliequivalents per liter. Intermediate values between 35 and 60 milliequivalents per liter may be seen in some CF patients (and in some normal children). In those cases, the sweat chloride test should be repeated in the very near future. In a severely malnourished patient with CF, the sweat chloride may be normal; however, once the malnutrition is corrected, the test will become positive. There are a few rare conditions which produce a false positive sweat chloride test. Such situations include diseases of adrenal, thyroid, or pituitary glands, rare lipid storage diseases, infection of the pancreas, etc. Generally, these children are easily differentiated from patients with CF by their clinical condition and molecular tests for CF can be done to clarify the situation.