Sweat electrolytes is a test that measures the level of chloride in sweat.

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.

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

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.

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 are as follows:

• Sodium:
  • Normal: less than 60 mEq/L
  • Abnormal: greater than 90 mEq/L
  • Equivocal: 60 - 90 mEq/L
• Chloride:
  • Normal: less than 40 mEq/L
  • Abnormal: greater than 60 mEq/L
  • Equivocal: 40 - 60 mEq/L

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.

An abnormal test may indicate the presence of cystic fibrosis.

Behrman RE. Nelson Textbook of Pediatrics. 17th ed. Philadelphia, Pa: WB Saunders; 2004.

LeGrys VA. Diagnostic sweat testing: the Cystic Fibrosis Foundation guidelines. J Pediatr. 2007;151(1):85-89.