Sweat Test for Cystic Fibrosis

The Gold Standard Diagnostic Test for Cystic Fibrosis

Photo © A.D.A.M.
Electrodes placed on arm for sweat test. Photo © A.D.A.M.

The sweat test has been the test of choice for diagnosing cystic fibrosis (CF) since 1959. Newer methods, such as genetic testing, have been developed but in most situations, the sweat test is still the test that leads to CF diagnosis. Some reasons for having a sweat test are:

What Happens During a Sweat Test?

The sweat test is usually done on the forearm, but may be done on the thigh of infants and children with small arms.

First Step - Stimulating Sweat: The technician will place two electrodes containing a sweat-inducing drug called pilocarpine on the skin. A small electric current is delivered through the electrodes to help the pilocarpine stimulate sweat. The current is not painful, but a slight tingling sensation may be felt.

Second Step – Collecting Sweat: After about 10 minutes, the technician stops the current and removes the electrodes. The skin is wiped dry and a piece of filter paper covered with plastic, sometimes called a “sweat patch” is placed on the site to collect the sweat. When a sufficient amount of sweat is collected, usually about 30 to 45 minutes, the sweat-drenched paper is removed and sent to the lab for testing.

What Does the Sweat Test Look For?

People with cystic fibrosis have high levels of salt, known as sodium chloride, in their sweat. The sweat test measures the amounts of sodium and chloride in the sweat, but the amount of chloride is the factor that determines the outcome of the test.

Chloride Ranges in Infants:

less than 30 = normal
30 – 59 = borderline
60 or greater = positive for CF

Chloride Ranges in Children and Adults:

less than 40 = normal
40 – 59 = borderline
60 or greater = positive for CF in children, but could still be normal for adults. If an adult has a result close to 60 mmol/liter, genetic testing may be done to confirm the diagnosis.

How Accurate is the Sweat Test?

The sweat test is 98% accurate, but some factors can cause the results to be incorrect or inconclusive.

False positives: Certain rare disorders can also produce high chloride levels in sweat, but those disorders have different symptoms than cystic fibrosis. If a sweat test produces positive results in a person who has symptoms that are not typical for CF, further testing should be done to confirm the diagnosis.

False negatives: Sometimes, a person with cystic fibrosis will have a normal chloride level in his or her sweat. If this happens, it is usually either because not enough sweat was provided in the sample, or the sweat was diluted.

False negatives are most often seen in:

  • Infants less than 2 weeks old or weighing less than 7 pounds who may not be able to produce enough sweat.
  • People with tissue swelling caused by fluid retention, known as edema, whose sweat may become diluted with tissue fluids.


Mishra, A., Greaves, R., and Massie, J. “The Relevance of Sweat Testing for the Diagnosis of Cystic Fibrosis in the Genomic Era”. 2005. Clin Biochem Rev. 26(4): 135–153. 20 July 2008.

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