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Sweat Test for Cystic Fibrosis
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Sweat Test for Cystic Fibrosis

A sweat test measures the amount of chloride in your sweat to help diagnose cystic fibrosis (CF), a condition that causes thick, sticky mucus to build up in the organs.

What is a sweat test?

A sweat test measures the amount of chloride in your sweat. Chloride is a type of electrolyte—an electrically charged mineral that helps control fluid levels and the balance of acids and bases (pH balance) in your body. Chloride and sodium form the salt found in your sweat.

Normally, chloride moves in and out of your cells through a protein called the cystic fibrosis membrane conductance regulator (CFTR). This protein is found in organs that produce sweat and mucus, such as the lungs, sweat glands, and intestines. If the CFTR protein does not function correctly, chloride cannot easily move in and out of cells, leading to cystic fibrosis (CF). In people with CF, chloride builds up in the cells, causing mucus to become thick and sticky, which can lead to breathing difficulties, frequent lung infections, and organ damage.

What is it used for?

A sweat test is used to diagnose cystic fibrosis. CF is an inherited disease that affects people of all ages. To have CF, a person must inherit a changed CFTR gene from both parents. While CF most commonly affects the lungs, it can also cause malnutrition by reducing the body's ability to absorb nutrients from food.

Why do I need a sweat test?

A health care provider may order a sweat test if they suspect you or your child has CF. It is most commonly performed on newborns following a positive newborn screening test. It is best to perform this test when a baby is between 10 days and 4 weeks old.

Providers may also order the test for children or adults who exhibit symptoms of CF or have a family history of the condition. Symptoms depend on which organs are affected and may include:

  • A wheeze or cough that may produce mucus or blood.
  • Clubbing of fingers and toes (fingernails and toenails that are wide and rounded).
  • Fever, which may include night sweats.
  • Digestive symptoms, such as severe abdominal pain, chronic diarrhea, constipation, or oily/loose stools.
  • Being underweight.
  • Muscle and joint pain.
  • Delayed growth or puberty.
  • Salty skin and saltier than normal sweat.
  • Sinus infections.
  • Infertility, especially in men.

What happens during a sweat test?

The collection process takes about an hour and involves the following steps:

  • The provider cleans and dries a small area of the forearm or leg.
  • An odorless chemical (pilocarpine) is applied to the area, and an electrode is taped over it.
  • A weak electrical current is sent through the electrode to help the chemical seep into the skin. This may cause a sensation of warmth or tingling.
  • After 5 minutes, the electrode is removed, and the sweat is collected using filter paper, gauze, or a small plastic coil.
  • Sweat is collected for 30 minutes and sent to a lab for analysis.

Will I need to do anything to prepare for the test?

No special preparation is required, but you should avoid applying any creams or lotions to the skin for 24 hours before the test.

Are there any risks to the test?

There are no known risks. The electrical current may cause a tingling or tickling sensation, but it is not painful.

What do the results mean?

Chloride levels are measured in mmol/L:

  • 60 or greater: Most likely indicates cystic fibrosis. Your provider may repeat the test or order genetic testing to confirm.
  • 30 to 59: The result is unclear, and CF is possible. Further testing, such as a repeat sweat test or genetic testing, is needed.
  • Less than 29: Normal. It is unlikely you have CF, though a normal result does not completely rule it out, as some people with CF have normal sweat test results.

Is there anything else I need to know about a sweat test?

In rare cases, conditions other than CF—such as hypothyroidism, nephrogenic diabetes insipidus, and Addison disease—may result in high chloride levels.

Frequently Asked Questions

You generally do not need to fast or stop medications before the test, but you should inform your provider about any recent illnesses or skin conditions. The procedure itself is painless; it involves placing a small electrode on the forearm or thigh to stimulate sweat production for about 30 to 45 minutes. While the electrical stimulation may feel like a mild tingling or itching, it does not cause pain, and the process is non-invasive.
chloride level of 60 millimoles per liter (mmol/L) or higher is typically diagnostic for cystic fibrosis. Levels between 30 and 59 mmol/L are considered borderline or intermediate, which may require repeat testing or genetic testing for confirmation. A level of 29 mmol/L or lower is generally considered normal and makes CF unlikely, though rare exceptions exist where genetic testing is still recommended if clinical suspicion remains high.
Yes, sweat tests can be performed on newborns, though the procedure requires more patience as infants may cry or move. Certain conditions can interfere with accurate results, including severe dehydration, eczema, or other skin disorders that affect sweat gland function. If the sweat sample is insufficient or the skin is irritated, the test may be inconclusive and need to be repeated after the skin has healed or the child is better hydrated.
negative sweat test does not completely rule out cystic fibrosis, especially in cases of atypical CF or specific genetic mutations that do not significantly alter chloride transport in sweat. Your provider may recommend genetic testing to look for specific CFTR gene mutations, or they may order additional diagnostic tests such as a fecal elastase test to check for pancreatic insufficiency or a nasal potential difference test.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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