The sweat chloride test is a simple test done to evaluate a patient who is suspected of having cystic fibrosis (CF) – the most common lethal genetic disease affecting Caucasians.
Cystic fibrosis is clinically suspected when the infant is slowly growing or when a toddler or young child is having recurrent serious intestinal or respiratory diseases.
During the test, a tiny (painless) electrical current is applied on the skin, usually on the patient’s forearm (in small infants on the back), to stimulate the maximum sweat production. This is called iontophoresis and it usually takes about one-half to one hour.
Then, sweat is collected from the skin by using specialized filter paper and the amount of chloride in the seat is measured because a patient with cystic fibrosis produces larger quantities of sweat chloride than normal individuals.
The normal sweat chloride values are 10-35 milliequivalents per liter and in patients with cystic fibrosis, a sweat chloride value is greater than 60 milliequivalents per liter.
The values between 35 and 60 milliequivalents per liter can be seen in some cystic fibrosis patients and in some normal children, but then a test should be repeated in the very near future.
The sweat chloride level can be normal in a severely malnourished patient with cystic fibrosis but once the malnutrition is corrected, the test becomes positive.
A test can be falsely positive in few rare conditions which include diseases of the adrenal, thyroid, or pituitary glands, rare lipid storage diseases, and infection of the pancreas but children with these diseases are easily differentiated from patients with cystic fibrosis by their clinical condition, and molecular tests for cystic fibrosis can be done to clarify the diagnosis.