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.

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.

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.

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

The sweat test usually takes 45 minutes to 1 hour.

No special preparation is needed before having 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, give them on the usual schedule.

You may help with the test and stay with your child during the test. If you can't stay, you may want to ask a family member or friend to stay with your child. Bring your child's favorite book or toy to help pass the time while the test is done. See if your child might be able to watch a movie during the test.

Results are usually available in 1 or 2 days.

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

Many conditions can change chloride levels. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.

The test results can help to confirm or rule out a diagnosis of cystic fibrosis.

High values:

• Usually mean that a person has cystic fibrosis. Some people with cystic fibrosis have borderline or even normal sweat chloride levels.
• May be caused by other conditions. But the sweat test is not used to diagnose these conditions, which include:
  • Adrenal glandproblems, such as adrenal insufficiency orAddison's disease.
  • Hypothyroidism.
  • Kidney failure.