Sweat test

Definition

A sweat test, sometimes called a sweat chloride test, is a procedure used to measure the amount of sodium and/or chloride (salt) excreted by a person's sweat glands.

Purpose

The sweat test is used to diagnosis cystic fibrosis (CF). CF is an incurable, inherited disorder that affects glands in the lungs, intestines, bile duct, and pancreas, as well as the sweat glands. The sweat test is administered as soon as CF is suspected, either because of family history or symptoms, such as frequent colds, recurrent lung infections, recurrent diarrhea , difficulty absorbing food, and slower-than-normal growth.

Because prompt diagnosis and treatment can often ease the severity of CF, sweat tests may be administered as early as the first week of life. This timing is recommended only when a family history of CF exists or the newborn exhibits symptoms specific to the disorder. However, newborns may not make enough sweat to accurately perform the test; hence, it may be repeated when they are older. Diagnosis of CF is made based on two or more sweat tests with abnormal chloride readings. Although sweat tests are highly accurate, diagnosis may be confirmed with genetic testing.

To have CF, a child must inherit a gene for the disorder from both parents. Because siblings of CF patients have a 25 percent chance of having the disorder, they should also be tested. However, the sweat test can determine only if the child has the disorder. It cannot determine whether a child is a carrier of a single CF gene that can be passed on to the next generation.

Description

Individuals with CF produce a higher than normal level of sodium chloride (salt) in their sweat. This measurement does not normally change with diet, medication, or environmental factors, making it a good diagnostic tool. The sweat test uses a process called iontophoresis. With iontophoresis, a very small, painless electric current is used to help draw sweat to the surface of the skin, where it can be collected and analyzed. The amount of electric current is tiny, and the test is safe and painless for all ages.

For infants a sweat test is done on the right thigh; for children and adults, the right forearm is used. After the area is washed and dried, two metal electrodes are attached and fastened with straps. Two gauze pads, one soaked in salt water or bicarbonate and the other in pilocarpine, a drug that stimulates sweating, are placed under the electrodes. A tiny electric current is applied to the skin for five to ten minutes to carry the pilocarpine into the skin. This stimulates the sweat glands to begin working. The procedure is painless; the child feels only a slight tingling or tickling.

After about ten minutes, the electrodes are removed. The skin is washed with distilled water and dried again. A dry piece of filter paper is taped to the area where the pilocarpine was applied. The paper is then covered with wax or a sheet of plastic, so that evaporation does not occur. The filter paper is called a sweat patch. After 30 to 45 minutes, the plastic is removed and the paper is placed in a sealed bottle. The entire process takes between 60 and 90 minutes.

It is important that the test be performed in a certified lab because the reliability of the test is operator-dependent. In the certified lab, the sweat patch is weighed and analyzed for sodium and/or chloride content. In children, normal sodium levels are less than 70 milliequivalents per milliliter (mEq/L). A sodium level greater than 90 mEq/L is indicative of CF. Normal chloride reading is less than 40 mEq/L. A chloride reading of greater than 60 mEq/L is indicative of CF. Readings falling between these numbers are borderline and require additional testing. Results are usually available within one to two working days.

Precautions

To ensure accuracy, sweat tests should be analyzed by a laboratory certified by the Cystic Fibrosis Foundation. Some other conditions such as malfunction of the adrenal gland or kidney failure can produce abnormal chloride readings. However, these conditions have distinct symptoms that differ substantially from CF. A sweat test is never used to diagnose these conditions.

Preparation

Before the sweat test, children should not exercise heavily or become overheated. There are no dietary restrictions; children may eat normally before the test. The results are not affected by medication; children may take their medication on the day of the test.

Aftercare

The test area may be red and sweaty for several hours after the test.

Risks

Although there is virtually no risk of electrical shock from a sweat test, it should never be conducted on the left side of the body, nor should it be given in the chest area, because there is a very small chance that the electric current could affect the heart. The current should come from a battery-powered unit rather than from a direct current.

Parental concerns

Parents can expect to stay with their child during the test.

KEY TERMS

Iontophoresis —Application of a small electric current to the skin.

See also Cystic fibrosis.

Resources

BOOKS

Bellet, Paul S. The Diagnostic Approach to Symptoms and Signs in Pediatrics, 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2002.

ORGANIZATIONS

Cystic Fibrosis Foundation. 6931 Arlington Road, Bethesda, MD 20814. Web site: <www.cff.org>.

WEB SITES

Goldenring, John. "Sweat Test." MedlinePlus Encyclopedia, 20 January 2004. Available online at <www.nim.nih.gov/medlineplus/ency/article/003630.htm> (accessed August 5, 2004).

"Sweat Testing." Living with CF. Cystic Fibrosis Foundation, April 2001. Available online at <www.cff.org/living_with_cf/sweat_testing.cfm> (accessed August 5, 2004).

Tish Davidson, A.M.