Sweat Test – Why am I having this test?

Sweat test, also called a chloride sweat test, is the standard test used to diagnose cystic fibrosis (CF). Your child may have this test if he or she has symptoms of CF, such as:

• Frequent respiratory infections, such as lung or sinus infections.
• Long-term (chronic) coughing or wheezing.
• Unusually salty sweat or skin.

In the U.S., all newborns are tested right after birth to check for the genes associated with CF. If your baby has the CF gene (has an abnormal screening result), the sweat test will be done to determine if your baby does or does not have CF. It is possible to have the CF gene but not have the condition. This is called being a “carrier.”

What is being tested?

This test measures the amount of chloride in a sample of sweat.

Chloride is a mineral that is naturally present in the body. Normally, only a small amount of chloride and salt (sodium) is in a person’s sweat. When your child has CF, sodium and chloride do not move properly in and out of the body’s cells. This causes a high level of sodium and chloride in your child’s sweat.

What kind of sample is taken?

A sweat sample is required for this test. To collect this sample, your child’s health care provider will:

1. Apply a small amount of liquid to an area of skin.
2. Place a sticky patch that has a wire connected to it (electrode) on the testing area. This causes the area to sweat. The testing area may feel warm and tingle.
3. Clean this area of skin after a few minutes.
4. Collect sweat from the testing area over a period of time. To collect the sweat, the health care provider may use:
   1. A plastic coil.
   1. Gauze.
   1. Filter paper.

The sweat sample will then be examined by the lab.

How do I prepare for this test?

Follow instructions from your child’s health care provider about when your child should stop using creams or lotions before the test. Your child may need to stop using certain creams and lotions starting 24 hours before the test.

Tell a health care provider about:

• Any allergies your child has.
• All medicines your child is taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
• Any blood disorders your child has.
• Any surgeries your child has had.
• Any medical conditions your child has.

How are the results reported?

Your child’s test results will be reported as a value that indicates how much chloride is in the sweat sample. This will be given as milliequivalents of chloride per liter of blood (mEq/L). Your child’s health care provider will compare your child’s results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals.

What do the results mean?

For children:

• Chloride level results:
  • 0–29 mEq/L is considered normal and means that CF is unlikely.
  • 30–59 mEq/L is still considered normal, but means that CF is more likely. Your child may need more testing, including a repeat of the sweat test.
  • 60 mEq/L or greater means that it is likely that CF is present.

It is possible to get a “quantity not sufficient” (QNS) result. This means the sample did not contain enough sweat to provide an accurate result. Your child may need to have the test repeated in this case.

If your child’s result indicates that CF is likely, your child will need more tests to confirm the diagnosis. Talk with your child’s health care provider about what the results mean.

Questions to ask your child’s health care provider

Ask your child’s health care provider, or the department that is doing the test:

• When will my child’s results be ready?
• How will I get my child’s results?
• What are my child’s treatment options?
• What other tests does my child need?
• What are my child’s next steps?

Summary

• A sweat test, also called a chloride sweat test, is the standard test used to diagnose cystic fibrosis (CF).
• This test measures the amount of chloride in a sample of sweat. The amount of chloride determines your child’s risk for CF.
• Make sure to discuss your child’s test results with your child’s health care provider. It is possible that your child may need more tests to confirm a diagnosis.