Children's Health

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.

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:
  • Adrenal gland problems, such as adrenal insufficiency or Addison's disease.
  • Hypothyroidism.
  • Kidney failure.

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.