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Cystic Fibrosis Sweat Test

Does this test have other names?

Chloride sweat test

What is this test?

A chloride sweat test is the gold standard test for diagnosing cystic fibrosis (CF). CF is a disease that causes mucus to build up and clog organs, especially the lungs, which causes breathing problems. CF can be life-threatening if untreated.

CF is an inherited disease that affects about 30,000 children and adults in the U.S. This simple test measures the amount of the chemical chloride, which is part of salt, in your child's sweat. This test is quite accurate because people with CF have more chloride in their sweat than people without this disease.

Why does my child need this test?

Although this test can diagnose CF in anyone, it's usually done on babies and young children. If your baby had a blood test at birth that was positive for CF, your baby's doctor may order a sweat test when your baby is 2 to 4 weeks old.

Your child's doctor may also order this test if he or she suspects CF because of signs or symptoms at any age. CF causes thick mucus to build up in the lungs and the digestive system. Signs and symptoms may include:

  • Salty-tasting skin

  • Coughing

  • Trouble breathing

  • Failure to gain weight

  • Frequent lung infections

  • Abnormal bowel movements 

What other tests might my child have along with this test?

Your child may also have a blood test or a swab of cells taken from inside the mouth to look for abnormal genes that cause CF.

What do my child's test results mean?

Many things may affect the lab test results. These include the method each lab uses to do the test. Even if the test results are different from the normal value, your child may not have a problem. To learn what the results mean for your child, talk with your child's health care provider.

Sweat test results are measured in units called millimoles per liter (mmol/L). Here are some possible readings for infants up to 6 months old:

  • 29 mmol/L or less means your baby probably does not have CF.

  • 30 to 59 mmol/L means your baby may have CF.

  • 60 mmol/L or higher means your baby has CF.

For anyone older than 6 months of age:

  • 39 mmol/L or less means the person is unlikely to have CF.

  • 40 to 59 mmol/L means the person may have CF.

  • 60 mmol/L or higher means the person has CF.

If your child has a sweat test done at a cystic fibrosis center, the caregivers at the center may discuss the results of the sweat test with you. The sweat test results may also be sent to your child's doctor, and you should discuss the results with him or her. 

How is this test done?

A sweat test takes about one hour. It is usually done at a CF care center. No needles are used for this test. Here is how it is done:

  • A clear liquid that causes sweating is rubbed over a small area of skin on the arm or leg.

  • An electrode is placed on the wet spot, and a small current of electricity is passed to the skin to stimulate sweat glands for about five minutes.

  • The small electric current may cause a mild tingling or warmth.

  • Sweat is then collected from the skin and sent off to have the amount of chloride in it measured.

Does this test pose any risks?

The electric current used is weak and causes no harm. Other than feeling a mild tingling or warmth, your child faces no risks with this test. In some cases, if too little sweat is collected or if the results are borderline, the test may need to be repeated.

What might affect my child's test results?

Some infants may not be able to make enough sweat to do this test. The test can be done at a later time. Once this test is positive, it will always be positive. Medications do not affect the results.

How do I get my child ready for this test?

The only preparation for the sweat test is to not use skin creams and lotions for 24 hours before the test.