Sweat Test for Cystic Fibrosis

Why this test may be ordered

• To diagnose cystic fibrosis in individuals with symptoms (e.g., persistent cough, salty skin, poor growth, frequent lung infections).
• To confirm a positive newborn screening result for cystic fibrosis.
• To evaluate whether a person with a family history of CF might have the disease.

Preparation and collection

You don't need any special preparations for a sweat test, but you should avoid applying any creams or lotions to the skin for 24 hours before the test.

To do a sweat test, a health care professional will collect a sample of sweat from you or your child. Sweat will be collected for 30 minutes. The sweat sample will be sent to a lab for testing. Some babies may not produce enough sweat for the test on the first try. If this happens, the test will usually be repeated on another day.

There is no known risk to a sweat test. The electrode may cause a tingling or tickling sensation from the electric current, but this is not painful.

Understanding results

Chloride in sweat is measured in mmol/L (millimoles of chloride per liter of sweat).

• Less than 29 mmol/L: A chloride level less than 29 is normal. This means that you or your child is unlikely to have CF. Limitation: But a normal test result doesn't rule out CF. Certain people with CF have normal sweat test results. Next step: If you or your child have symptoms of CF or if your baby had a newborn screening test that showed possible CF, your provider may order more tests.
• 30 to 59 mmol/L: A chloride level of 30 to 59 means that more testing is needed because the test result is unclear, and CF is possible. Limitation: In rare cases, conditions other than CF may result in high chloride levels on a sweat test. Next step: Your provider may order another sweat test or a different type of test, such as genetic testing.
• 60 mmol/L or greater: A chloride level of 60 or greater means that you or your child most likely has cystic fibrosis. Limitation: In rare cases, conditions other than CF may result in high chloride levels on a sweat test. These conditions include hypothyroidism, nephrogenic diabetes insipidus, and Addison disease. Next step: To confirm the diagnosis, your provider may have you repeat the test or order a CF genetic test.

• A normal sweat chloride level (<29 mmol/L) does not completely rule out cystic fibrosis; some people with CF have normal results.
• A positive sweat chloride level (≥60 mmol/L) may rarely be caused by conditions other than CF, such as hypothyroidism, nephrogenic diabetes insipidus, or Addison disease.
• Results between 30 and 59 mmol/L are considered borderline and require further testing.
• There is no known risk from the sweat test itself; the electrical current may cause a mild tingling or warmth.

What to do next

If you or your child have symptoms of CF or if your baby had a newborn screening test that showed possible CF, your provider may order more tests.

• What do my sweat chloride results mean for my or my child's health?
• If the result is borderline, what additional tests should I expect?
• How soon should follow-up testing be done if the result is positive or borderline?

Evidence source

Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.

U.S. National Library of Medicine: Sweat Test for Cystic Fibrosis

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