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
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.
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.
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.
There is very little risk of complications from
this test. But the test should always be done on an arm or leg (not the chest)
to prevent the possibility of electric shock.
The electric current
may cause skin redness and excess sweating for a short time after the test is
done. In rare cases, the current may make the skin look slightly
A sweat test measures the amount of salt
chemicals (sodium and chloride) in sweat. Generally, chloride (sweat chloride)
Results are usually available in 1 or 2 days. Normal
results vary from lab to lab.
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 4 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.
skin rash or sore on the area of the skin where the gauze pads are
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. For more information, see the
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
Farrell PM, et al. (2008). Guidelines for diagnosis of
cystic fibrosis in newborns through older adults: Cystic Fibrosis Foundation
consensus report. Journal of Pediatrics, 153(2):
Other Works Consulted
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
LeGrys VA, et al. (2007). Diagnostic sweat testing:
The Cystic Fibrosis Foundation Guidelines. Journal of Pediatrics, 151(1): 85–89.
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.
How this information was developed to help you make better health decisions.