What is a sweat test?

A sweat test measures the amount of chloride in sweat. It can help diagnose cystic fibrosis. People with cystic fibrosis have a high level of chloride in their sweat.

During the sweat test, medicine that causes a person to sweat is applied to the skin. This is usually done on the arm or thigh. The sweat is collected and the amount of chloride is measured in a lab.

Your child may need more than one sweat test to confirm a diagnosis of cystic fibrosis.

Why is it done?

The sweat test is done to help diagnose cystic fibrosis. It is often done after a baby tests positive for cystic fibrosis on a newborn screening test. It also may be used to test children with a family history of cystic fibrosis and for anyone with symptoms of cystic fibrosis.

How do you prepare for the test?

You don't need to do anything to prepare before your child has this test. But avoid using creams or lotions on the skin for 24 hours before the test. Your child may eat, drink, and exercise normally before the test. If your child takes any medicines, they can take them on the usual schedule.

How is the test done?

The sweat test is usually done on a baby's arm or thigh. On an older child or adult, the test is usually done on the inside of the forearm. Sweat is usually collected and analyzed from two different sites.

1. The skin is washed and dried, and then two small gauze pads are placed on the skin. One pad is soaked with a medicine that makes the skin sweat.
2. Other pads, called electrodes, are placed over the gauze pads. The electrodes are hooked up to a device that produces a mild electric current. This pushes the medicine into the skin.
3. After 5 to 10 minutes, the gauze pads and electrodes are removed. The skin is cleaned with water and dried. The skin will look red in the area under the pad that contained the medicine.
4. A dry gauze pad, a paper collection pad, or special tubing is taped to the red patch of skin. The pad may be covered with plastic or wax. This helps prevent fluid loss (evaporation).
5. The new pad will soak up the sweat for up to 30 minutes. Then it's removed and placed in a sealed bottle. It's then weighed to measure how much sweat the skin produced. It's also checked to find out how much salt chemical (sodium, chloride, or both) the sweat contains. Another kind of test collects the sweat into a coil (macroduct technique).
6. After the collection pad is removed, the skin is washed and dried again. The test site may look red. It may continue to sweat for several hours after the test.

This test does not cause pain. Some children feel a light tingling or tickling when the electric current is applied to the skin.

How long does the test take?

The sweat test usually takes 45 minutes to 1 hour.

What happens after the test?

Your child will probably be able to go home right away. Your child can go back to their usual activities right away.