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Sweat test (chloride test for cystic fibrosis)
Category: Biopsies, Lab & Pathology Tests · All biopsies, lab & pathology tests
What it is
The sweat test measures the salt (chloride) concentration in sweat, which is characteristically high in cystic fibrosis (CF). A mild electrical current with a medicine (pilocarpine) stimulates sweating on a small area of skin, and the sweat is collected and analysed.
Why it is done
It is the standard confirmatory test when CF is suspected — a positive newborn screening result, failure to thrive, recurrent chest infections or characteristic symptoms at any age.
How to prepare
The baby or child should be well hydrated and without eczema or broken skin at the test site. Tell the team about recent illness and dehydration, which can affect sweat production. No fasting is needed and the test is painless.
What happens during the procedure
Electrodes with pilocarpine gel are strapped to the arm or leg for about five minutes (a mild tingling), then a collection device or gauze gathers sweat for about 30 minutes. The laboratory measures the chloride concentration and the amount of sweat collected (too little sweat means the test must be repeated).
Afterwards — recovery & results
Results are usually available within days: chloride levels are reported as normal, borderline (intermediate — needing repeat and genetic testing) or consistent with CF. Your specialist team explains the result and arranges confirmatory genetics and referral to a CF centre if needed.
Risks & considerations
Risks are minimal: mild redness or, very rarely, a small burn from the electrodes. An insufficient sample is the commonest practical problem, particularly in small babies, and simply means repeating the test.
Related guides on MedicalVideos.com
Frequently asked questions
Why is sweat salty in cystic fibrosis?
The faulty CF channel normally reabsorbs salt from sweat as it travels to the skin surface. Without it working, sweat keeps its salt — the basis of the test and of the old saying about a child who 'tastes salty'.
A borderline result — what happens?
The test is repeated and genetic testing for CF variants is done. Some people carry CF-related conditions with intermediate results, which specialist teams assess individually.
Related procedures — Biopsies, Lab & Pathology Tests
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