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Sweat Test: About Your Child's Test

Electrodes connected to the arm to produce sweat, and a device on the arm that collects sweat.

What is a sweat test?

A sweat test measures the amount of salt chemicals (sodium and chloride) in sweat. It can help diagnose cystic fibrosis. People with cystic fibrosis have 2 to 5 times the normal amount of sodium and 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 on a paper or gauze pad. Then the amount of salt chemicals in the paper or gauze is measured in a lab. In most cases, chloride (sweat chloride) is measured.

Your child may need more than one sweat test to confirm whether he or she has cystic fibrosis.

Why is it done?

The sweat test is done to help diagnose cystic fibrosis. It also may be used to test children with a family history of cystic fibrosis and for anyone with symptoms of cystic fibrosis.

How can you prepare for the test?

You don't need to do anything to prepare before your child has this test. Your child may eat, drink, and exercise normally before the test. If your child takes any medicines, he or she can take them on the usual schedule.

What happens during the test?

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 his or her usual activities right away.

Follow-up care is a key part of your child's treatment and safety. Be sure to make and go to all appointments, and call your doctor or nurse call line if your child is having problems. Ask your doctor when you can expect to have your child's test results.

Where can you learn more?

Go to https://www.healthwise.net/patientEd

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