The sweat test is a simple, painless diagnostic test used to determine whether someone has cystic fibrosis (CF). It works by measuring the amount of salt in the person’s sweat, and is considered to be the gold standard in diagnosing this disease.
Specifically, the sweat test assesses the amount of chloride in sweat. Chloride is a salt ion — half of “sodium chloride,” better known as table salt.
In CF, a genetic mutation leads to defects in a protein called CFTR. This protein normally functions like a “gate” on the surface of certain cells, controlling the movement of chloride and water in and out of the cell. The lack of functional CFTR protein leads to irregularities in chloride transport, which is what leads to the formation of the thick, sticky mucus that characterizes CF.
CFTR also is present in the skin, so measuring the salt content of sweat is a way to easily test the function of CFTR without a painful or invasive test. Due to the lack of functional CFTR, people with cystic fibrosis have abnormally high amounts of chloride in their sweat. Chloride levels in sweat from CF patients can be two to five times higher than is normal.
The sweat test is often 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 avoiding putting any creams or lotions on the test area for 24 hours beforehand. No injections are used in a sweat test, which usually takes about an hour to perform.
At the start of the test, a chemical called pilocarpine is applied to a small area of skin, typically on the arm or leg, to induce sweating.
An electrode is then placed on the skin, and a weak electric current is applied for about five minutes. The purpose of the electricity is to ensure the pilocarpine is promptly absorbed into the skin. The electric current does not cause pain, though some people experience feelings of tingling and warmth.
After the electrode is removed, a piece of filter paper or gauze is put on the test area to collect the sweat, and is generally left for about 30 minutes. The collected sweat is then sent to a laboratory to test its chloride content.
There are variations on this setup: for example, in some cases, two electrodes are used, one of which contains pilocarpine in the form of a gel. The overall concept of the test is the same.
The sweat test has minimal risk of safety problems or complications. In some cases, the tested area may redden or appear sunburned; this usually resolves after a few days.
The time it takes to get the results of the sweat test varies, but is usually only a few days. Healthcare providers will discuss the results with the patient and the family, as well as treatment options that are available if the patient tests positive for cystic fibrosis.
Generally, sweat chloride levels of 60 millimole per liter (mmol/L) or higher are highly suggestive of CF, whereas levels below 30 mmol/L indicate that a person probably doesn’t have CF. Levels between 30 and 59 mmol/L indicate that a person may have CF.
If the results of a sweat test are inconclusive, the test may be repeated. Other tests, especially genetic testing, may be done to confirm the diagnosis.
Last updated: Sept 23, 2021
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