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

Avoid applying creams or lotions to the skin for 24 hours before the test.

Sweat is collected by applying pilocarpine and a weak electrical current to the forearm or leg, then absorbing sweat onto filter paper or gauze for 30 minutes.

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

Understanding results

The sweat test measures the amount of chloride in your sweat. Results are reported in millimoles per liter (mmol/L).

• Less than 29 mmol/L: It is unlikely that you or your child has cystic fibrosis. Limitation: A normal result does not completely rule out cystic fibrosis. If symptoms persist or newborn screening was positive, your provider may recommend additional testing. Next step: No further action is needed unless symptoms develop or screening results suggest otherwise.
• 30 to 59 mmol/L: Cystic fibrosis is possible but not certain. More testing is needed. Limitation: This result is unclear and does not confirm or exclude cystic fibrosis. Further testing is necessary. Next step: Your provider will likely order another sweat test or genetic testing to clarify the diagnosis.
• 60 mmol/L or greater: You or your child most likely has cystic fibrosis. Limitation: A high chloride level strongly suggests cystic fibrosis, but confirmatory testing (repeat sweat test or genetic testing) is usually recommended. Next step: Your provider may order a repeat sweat test or a CF genetic test to confirm the diagnosis and guide treatment.

• 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

Based on the results, your healthcare provider may recommend repeating the sweat test, genetic testing, or other evaluations to confirm or rule out cystic fibrosis.

• 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

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