Test Overview
A sweat
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
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?). 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
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.
Risks
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
sunburned.
Results
A sweat test measures the amount of salt
chemicals (sodium and chloride) in sweat. Generally, chloride (sweat chloride)
is measured.
Results are usually available in 1 or 2 days. Normal
results vary from lab to lab.
Sweat chloride in people age 6 months and older
1
| Normal: |
Less than 40
millimoles per liter (mmol/L)
|
| Borderline: |
40–59 mmol/L
|
| Abnormal: |
60 mmol/L or more
|
Sweat chloride in babies younger than 6 months
1
| Normal: |
Less than 30 mmol/L
|
| Borderline: |
30–59 mmol/L
|
| Abnormal: |
60 mmol/L or more
|
Many conditions can change sodium and chloride levels.
Your doctor will discuss any significant abnormal results with you
in relation to your symptoms and past health.
The test results
do not show how severe the cystic fibrosis is. The test only shows if a
person could have the disease.
Abnormal (high) values
High values:
- Usually mean 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:
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 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.
- 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
- Usually, two sweat tests are done 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. For more information, 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. For more information, 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.
References
Citations
-
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): S4–S14.
Other Works Consulted
- 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.
Credits
|
By
|
Healthwise Staff |
|
Primary Medical Reviewer
|
John Pope, MD - Pediatrics |
|
Specialist Medical Reviewer
|
Susanna McColley, MD - Pediatric Pulmonology |
|
Last Revised
|
June 15, 2011 |