Sweat Test Health Article

Sweat Test

A test, used in the diagnosis of cycstic fibrosis, that measures the level of sodium and chloride excretions from the sweat glands.

The sweat test is used in the diagnosis of cystic fibrosis (CF), an incurable, inherited disease that affects the sweat glands, as well as glands in the lungs, intestines, bile duct, and pancreas. The test is administered as soon as CF is suspected, either because of a family history or symptoms such as frequent colds, recurrent lung infections, recurrent diarrhea, difficulty absorbing food, and slower-than-normal growth. Because early diagnosis and treatment can often ease the severity of the disease, sweat tests may be administered as early as the first week of life. However, this is recommended only when a family history of CF exists or symptoms specific to the disease are exhibited. Sweat tests should never be given for minor breathing problems. Because siblings of CF patients carry a 25% chance of having the disease, they should also be tested.

For infants, the test is done on the right thigh; for children, 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 and the other in pilocarpine to induce sweating, are placed under the electrodes. A tiny electric current is sent for five to 10 minutes to carry the pilocarpine into the skin. The child should only feel a slight tingling or tickling. 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. The current should come from a battery-powered unit rather than from a direct current.

After the electrodes are removed, the skin is cleansed and dried again. A dry gauze pad or piece of filter paper covered with a sheet of clear plastic is taped to the area where the pilocarpine was applied. The gauze or filter paper, called a sweat patch, is weighed before application. After 30-45 minutes, the plastic is removed and the gauze or paper is placed in a sealed bottle. The test area may be red and sweaty for several hours after the test. In the lab the sweat patch will be weighed and analyzed for sodium and chloride content. A sodium level above 90 milliequivalents per milliliter and a chloride level above 60 is indicative of cystic fibrosis.