Sweat Test

Why It Is Done

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

How To Prepare

No special preparation is needed before having this test. Your child may eat, drink, and exercise normally before the test. If your child takes any medicines, he or she may take 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.

Talk with your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).

How It Is 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.

• 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 salt chemical (sodium and/or 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.

How It Feels

This test does not cause pain. Some children feel a light tingling or tickling when the electric current is applied to the skin. If the gauze pads are not properly placed, the electric current may produce a burning sensation.

What Affects the Test

Reasons your child may not be able to have the test or why the results may not be helpful include:

• A baby's age. Babies younger than 2 weeks may not produce enough sweat to give reliable test results and may have lower sweat chloride levels than older babies and children. A minimum amount of sweat is needed for accurate test results regardless of the child's age.
• A skin rash or sore on the area of the skin where the gauze pads are attached.
• Acute or severe illness.
• Dehydration or heavy sweating.
• Decreased sweating.
• Normal fluctuations in sodium and chloride during puberty.
• A decrease in the hormone aldosterone.
• Steroid medicines, such as fludrocortisone.

What To Think About

• Your child may need more than one sweat test to confirm a diagnosis of cystic fibrosis.
• A sweat test can't identify carriers of the cystic fibrosis gene. If your child is diagnosed with cystic fibrosis, you may wish to talk with your doctor about genetic counseling. To learn more, see the topic Cystic Fibrosis Carrier Screening.
• Adults generally have higher salt concentrations in their sweat than children. Also, sweat test results in adults can vary widely. This is especially true in women, because the amount of salt in their sweat can vary with the phase of their menstrual cycle. Enough sweat must be collected to get accurate test results.
• If results of a sweat test are positive or unclear (especially in babies), a blood test may be done to detect changes in the genetic material (DNA) that causes cystic fibrosis. Blood test results are usually ready in 10 to 21 days. To learn more, see the topic Genetic Test.
• Sweat tests should be done at labs that are certified by the Cystic Fibrosis Foundation (www.cff.org). These labs perform a large number of sweat tests and are skilled at sweat test techniques and interpretation.