Definition

Sweat electrolytes is a test that measures the level of chloride in sweat. Although genetic tests have become important methods for determining whether a child has cystic fibrosis, the sweat chloride test remains important.

How the test is performed

In the first part of the test, a colorless, odorless chemical that causes sweating is applied to a small area on an arm or leg. An electrode is then attached to the arm or leg, which allows the technician to apply a weak electrical current to the area to stimulate sweating.

People may feel a tingling sensation in the area, or a feeling of warmth. This part of the procedure lasts approximately 5 minutes.

The next part of the test involves cleaning the stimulated area and collecting the sweat on a piece of filter paper or gauze, or in a plastic coil. After 30 minutes, the collected sweat is sent to a hospital laboratory for analysis. The entire collection procedure takes about 1 hour.

How to prepare for the test

No special preparation is necessary. Make sure the center where the test is being performed is a cystic fibrosis testing center.

How the test will feel

Though the test is not painful, some people describe a tingling sensation at the site of the electrode. In smaller children or infants, the sensation can cause irritability or discomfort.

Why the test is performed

Sweat testing is the standard method for diagnosing cystic fibrosis. People with cystic fibrosis have higher amounts of sodium and chloride in their sweat, which the test can detect.

Some people are referred for testing because of symptoms such as poor growth, many respiratory infections, or foul-smelling stools. In some states, newborn screening programs test for cystic fibrosis, and the sweat test is used to confirm these results.

Normal Values

• A sweat chloride test result less than or equal to 39 mEq/L in an infant over 6 months old probably means cystic fibrosis is not present.
• A result between 40 - 59 mEq/L does not give a clear diagnosis. Further testing is needed.
• If the result is 60 mEq/L or greater, cystic fibrosis is present.

Note: mEq/L = milliequivalent per liter

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

An abnormal test may indicate the presence of cystic fibrosis.

What the risks are

Special considerations

References

Boat TF, Acton JD. Cystic fibrosis. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 400.

Farrell PM, Rosenstein BJ, White TB, et al. Guidelines for diagnosis of cystic fibrosis in newborns through older adults: Cystic fibrosis consensus report. Journal of Pediatrics. Aug 2008;153(2).