The sweat test is a simple, painless diagnostic test used to measure the salt content of sweat to determine whether someone has cystic fibrosis. It’s considered by clinicians to be the gold standard in diagnosing this disease.
About cystic fibrosis
CFTR is a protein at the cell membrane that transports salt (chloride) and water in and out of the cell. It is present on the surface of most cells.
Lacking fully functional CFTR protein results in the buildup of a thick mucus around major organs, especially in the lungs, where it can interfere with function and make the lungs more prone to infection.
CFTR is also present in the skin, so measuring the salt content of sweat allows doctors to evaluate the function of CFTR without a painful or invasive test.
About the sweat test
Patients with cystic fibrosis have more chloride in their sweat due to the lack of functional CFTR protein. The sweat test is used to confirm a diagnosis of cystic fibrosis after a positive newborn screening for CF. The test is also frequently used in clinical trials as a measure to evaluate whether a particular treatment improves CFTR function.
How the sweat test is performed
No special preparation is needed prior to the test, other than to avoid treating the test area with any creams or lotions for 24 hours before the test, which takes about an hour. No injections are used in this procedure.
A small area of skin on the arm is washed and dried. Two electrodes are attached to the arm with straps. One of these electrodes has a disk containing pilocarpine gel, which increases the production of sweat.
A weak electric current pushes the treatment through the skin for about five minutes. Some patients have said they feel a mild tingling or itching under the electrode.
The electrodes are then removed, and a sweat collection device is strapped to the arm for 30 minutes. After the sweat has been collected, the device is removed and placed in an analyzer.
The patient’s arm may turn red in the treated area and may continue sweating for several hours after the test. Infants may not produce enough sweat during the test to get a result. When this happens, the test must be repeated.
The time it takes to get the results of the sweat test varies, but parents or patients usually receive the results within a few days. The doctor will discuss the results with the patient or their family, as well as treatment options that are available if the patient tests positive for cystic fibrosis.
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