Sweat Test for Cystic Fibrosis

What is a sweat test?

A sweat test measures the amount of chloride in sweat. Chloride is part of the salt that's found in sweat. The test can diagnose cystic fibrosis (CF) because people with CF have higher levels of chloride in their sweat.

CF is an inherited disease that can affect people of all ages. To have cystic fibrosis, you must inherit a changed CFTR gene from both your parents. (Changes in genes are also called variants or mutations.) If you inherit one changed gene, you won't have the disease, but you will be a carrier. Being a carrier means that if you have children with someone who also has changes in one or both of their CFTR genes, your children could inherit two changed genes and have CF.

The disease affects the parts of your body that make mucus and sweat. Normally, mucus is slippery and protects the linings of your airways, digestive tract, and other organs and tissues. With CF, mucus becomes thick and sticky. It can build-up in your organs and cause damage.

CF commonly affects the lungs. Thick mucus may make it hard to breathe and can lead to frequent lung infections and damage. Another common problem from CF is malnutrition because the disease may reduce your body's ability to absorb nutrients from food.

Other names: sweat chloride test, cystic fibrosis sweat test, sweat electrolytes

What is it used for?

A sweat test is used to diagnose cystic fibrosis.

Why do I need a sweat test?

A sweat test can diagnose cystic fibrosis (CF) in people of all ages:

• Babies in the United States have newborn screening tests to check for a variety of conditions, including CF. If a screening test shows that your baby may have CF, a sweat test will be needed to make a diagnosis. It's best if the test is done when your baby is between 10 days and 4 weeks old. Starting treatment as soon as possible can delay or prevent health problems from CF.
• Children and adults may need a cystic fibrosis sweat test if they have symptoms of CF or if someone in the family has CF. Most people with CF have symptoms, but some people don't. In milder cases, symptoms may not show up until adulthood. The symptoms of CF depend on which organs are affected and may include:
  • A wheeze or cough that may produce mucus or blood.
  • Clubbing of fingers and toes, which means that your fingernails and toenails are wide and rounded like the back of a spoon. This is usually due to a low level of oxygen in your blood that happens if your lungs aren't working well. The lack of oxygen may cause changes in your nails.
  • Fever, which may include night sweats (heavy sweating during sleep).
  • Digestive symptoms, such as severe abdominal (belly) pain, chronic (long-lasting) diarrhea, or constipation.
  • Being underweight.
  • Muscle and joint pain.
  • Delayed growth or puberty.
  • Salty skin and saltier than normal sweat.
  • Sinus infections.

If your health care provider suspects CF, it's important to get tested. There is no cure for CF, but treatment can help you and/or your child live a longer, healthier life.

What happens during a sweat test?

To do a sweat test, a health care professional will collect a sample of sweat from you or your child. This takes about an hour and usually includes the following steps:

• To cause sweating, an odorless chemical (pilocarpine) will be applied to a small area of the forearm or leg. An electrode will be taped over the area.
• A weak electrical current will be sent through the electrode to make the chemical seep into the skin. This may cause a little tingling or warmth.
• After about 5 minutes, the electrode will be removed. The health care professional will collect the sweat by taping a piece of filter paper or gauze on the area or by using a small plastic coil.
• 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.

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

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.

Are there any risks to the test?

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.

What do the results mean?

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

• A chloride level of 60 or greater means that you or your child most likely has cystic fibrosis. To confirm the diagnosis, your provider may have you repeat the test or order a CF genetic test.
• A chloride level of 30 to 59 means that more testing is needed because the test result is unclear, and CF is possible. Your provider may order another sweat test or a different type of test, such as genetic testing.
• A chloride level less than 29 is normal. This means that you or your child is unlikely to have CF. But a normal test result doesn't rule out CF. Certain people with CF have normal sweat test results. If you or your child have symptoms of CF or if your baby had a newborn screening test that showed possible CF, you will probably need more testing.

Learn more about laboratory tests, reference ranges, and understanding results.

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

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.