test measures the amount of salt chemicals (sodium and chloride) in sweat. It
is done to help diagnose
cystic fibrosis. Normally, sweat on the skin surface
contains very little sodium and chloride. 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 (usually on the arm or thigh). The sweat is then collected on a
paper or a gauze pad, and the amount of salt chemicals in the paper or gauze is
measured in a lab. Generally, chloride (sweat chloride) is measured. See a
picture of a
sweat test .
A sweat test is done on any person suspected of having
cystic fibrosis. An initial test may be done as early as 48 hours of age. But a
sweat test done during the first month of life may need to be repeated. Younger
babies may not produce enough sweat to give reliable test results. Also,
younger babies may naturally have lower sweat chloride levels than older babies
and children with cystic fibrosis.
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
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?). For more information, see the topic Pediatric Preparation for Medical Tests.
How It Is Done
The sweat test is usually done on a
baby's right arm or thigh. On an older child or adult, the test is usually done
on the inside of the right 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
- 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
- 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).
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
The sweat test usually takes 45 minutes to 1 hour.