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.

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?).

To learn more, see the topic Pediatric Preparation for Medical Tests.

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.

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
Normal:

Less than 30 millimoles per liter (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 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:

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.

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • 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

ByHealthwise Staff
Primary Medical Reviewer John Pope, MD – Pediatrics
Kathleen Romito, MD – Family Medicine
Specialist Medical Reviewer R. Steven Tharratt, MD, MPVM, FACP, FCCP – Pulmonology, Critical Care Medicine, Medical Toxicology

Current as ofMay 4, 2017