Sweat Test for Cystic Fibrosis: How It Works and What Results Mean

Nov 7

4 min read

Updated: 3 days ago

Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: November 2026 · How we review

Quick Answer

The sweat test (sweat chloride test) is the main test used to diagnose cystic fibrosis (CF). It measures how much chloride (salt) is in the sweat. A chloride level above 60 mmol/L means CF is likely, below 40 means CF is unlikely, and 40 to 60 may need repeat and genetic testing.

What the research says

The sweat test is the cornerstone of CF diagnosis: international Cystic Fibrosis Foundation consensus guidelines recommend that a CF diagnosis in anyone, from newborns to adults, be confirmed with a sweat chloride test (Farrell PM et al., J Pediatr, 2017) [2]
Where you land in the middle range matters: among infants in Indiana with a positive newborn screen, one CFTR gene change and an intermediate first sweat test (30–59 mmol/L), none with 30–39 mmol/L and only 1 of 31 with 40–49 mmol/L were later diagnosed with CF, compared with 61% of those at 50–59 mmol/L (Bauer SE et al., Pediatr Pulmonol, 2021) [3]

Key takeaways

The most reliable test for diagnosing cystic fibrosis.
Measures chloride in sweat; levels are high in CF.
Painless, needle-free, with about 30 minutes of sweat collection.
Higher than 60 mmol/L suggests CF; intermediate results need repeat or genetic testing.
Babies are usually tested at about 2–4 weeks old.

What is cystic fibrosis?

Cystic fibrosis is an inherited condition that causes thick, sticky mucus to build up in the lungs, pancreas and other organs. A faulty CFTR protein prevents chloride from moving normally through cells, so salt collects in sweat. That's why measuring sweat chloride is so useful. Sources: [1]

Why is it done?

List of reasons for a sweat test including a positive newborn screen, family history of cystic fibrosis, salty skin and poor weight gain

The sweat test confirms or rules out cystic fibrosis.

After a positive newborn screening test for CF.
When a brother, sister or other relative has CF.
When a child has salty-tasting skin, poor weight gain despite a good appetite, or greasy, bulky stools.
For repeated lung or sinus infections, a lingering cough or wheezing.
For adults with symptoms that could be a milder form of CF, including recurrent pancreatitis. Sources: [1, 2]

Other tests may include genetic testing, chest and sinus X-rays and pulmonary function tests. Sources: [1]

How do I prepare?

There are no limits on eating, activity or medicines. Don't put creams or lotions on the skin before the test. Newborns may not make enough sweat in the first days of life, so babies are usually tested at about 2–4 weeks old. Sources: [1]

What happens during the test?

Four steps of the sweat test: pilocarpine, mild current, sweat collection and lab analysis

The sweat test uses no needles.

Pilocarpine, a sweat-stimulating medicine, is placed on a small area of the arm or leg.
A mild electric current helps the medicine enter the skin; you may feel slight tingling.
Sweat is collected on filter paper or gauze, or in a plastic coil, for about 30 minutes.
The sample goes to a lab, which measures the chloride level. Sources: [1]

The test isn't painful. Sometimes too little sweat is collected, and the test needs to be repeated. Sources: [1]

Understanding your results

Three cards explaining sweat chloride results: CF likely, intermediate and CF unlikely

Intermediate results need follow-up.

Sweat chloride (Cleveland Clinic)	What it means
Above 60 mmol/L	CF is likely
40–60 mmol/L	Borderline; repeat testing and other tests may be needed
Below 40 mmol/L	CF is unlikely

Newer Cystic Fibrosis Foundation guidelines use 60 mmol/L or more as consistent with CF and 30–59 mmol/L as intermediate. Intermediate results usually lead to a repeat sweat test and genetic testing for CFTR gene changes. Sources: [1, 2, 3]

The test measures chloride, the same mineral checked in a chloride blood test, but blood chloride doesn't diagnose CF. Sources: [1]

Conclusion

The sweat test is a simple, painless and reliable way to confirm or rule out cystic fibrosis. Clear results give an answer quickly, while borderline results are followed up with repeat and genetic testing so treatment can start early when needed.

Keep learning: read about cystic fibrosis and genetic testing.

Frequently asked questions

What does a sweat test measure?

The amount of chloride in your sweat.

Is the sweat test painful?

No. It uses no needles; you may feel slight tingling.

How long does it take?

Sweat is collected for about 30 minutes.

What result means cystic fibrosis?

Above 60 mmol/L suggests CF is likely.

What is an intermediate result?

A middle-range level that needs repeat or genetic testing.

Why do babies wait until 2–4 weeks?

Newborns may not make enough sweat in the first days.

Do I need to prepare?

Avoid creams or lotions on the skin beforehand.

Can adults have a sweat test?

Yes. People of any age can be tested.

Why might the test be repeated?

If too little sweat was collected or results are borderline.

Is a blood chloride test the same?

No. Only sweat chloride is used to diagnose CF.

References

Cleveland Clinic. Sweat Test for Cystic Fibrosis. Cleveland Clinic. Last updated February 14, 2019. https://my.clevelandclinic.org/health/diagnostics/17856-sweat-test-for-cystic-fibrosis-cf
Farrell PM, White TB, Ren CL, et al. Diagnosis of cystic fibrosis: consensus guidelines from the Cystic Fibrosis Foundation. J Pediatr. 2017;181S:S4-S15.e1. PMID 28129811. https://doi.org/10.1016/j.jpeds.2016.09.064
Bauer SE, Wesson M, Oles SK, et al. Outcomes of repeat sweat testing in cystic fibrosis newborn screen positive infants. Pediatr Pulmonol. 2021;56(6):1521-1526. PMID 33512069. https://doi.org/10.1002/ppul.25296

Medical disclaimer

This article is for general education only and does not replace advice from a qualified healthcare professional. Talk to your doctor about your own diagnosis and treatment, and seek emergency care right away for severe or sudden symptoms.