Sweat Test

The sweat test has been the “gold standard” for diagnosing cystic fibrosis (CF) for more than 40 years. When it is performed by trained technicians, and evaluated in an experienced, reliable laboratory, the sweat test is still the best test to diagnose CF. It is recommended that the sweat test be performed in a Cystic Fibrosis Foundation-accredited care center where strict guidelines are followed to ensure the accuracy of the results.

The test only is performed on individuals more than 10 days old. However, some infants may not make enough sweat for the laboratory to analyze. If an infant does not produce enough sweat on the first sweat test, it should be repeated after 7 days.

Patients can be seen by Texas Children's experts in Cystic Fibrosis Care Center.

How to schedule a sweat test

The patient’s physician places an order that can be in either Epic or faxed to the TCH Sweat Test lab at 832-825-5162. Parents can then call 832-824-5152 to schedule.

Infants meeting the following criteria may undergo sweat testing:

• Minimum weight of 4.4 pounds
• Greater than 10 days old
• If the patient has any of the following, it cannot be scheduled:
• Acute Diarrhea
• Dehydration
• Severe Eczema on both arms or burns, allergic reaction, or non-intact skin.
• Fever
• Vomiting
• If the patient is receiving oxygen by an open delivery system
• If the patient has an IV in both arms. 

A list of medication that the patient is on must be provided. Sweat collection may be compromised in patients on injections of Hydrocortisone, Prednisone, and Beclomethasone. If the patient is on any of these medications, our Chemistry Pathologist will review and make a final decision if the test can be scheduled.

What happens during a sweat test?

The sweat test determines the amount of chloride in the sweat. There are no needles involved in the procedure. In the first part of the test, a colorless, odorless chemical, known to cause sweating, is applied to a small area on an arm, both arms will be tested. An electrode is then attached to the arm one at a time, which allows the Technologist to apply a weak electrical current to the area to stimulate sweating. Individuals may feel a tingling sensation in the area, or a feeling of warmth. This part of the procedure lasts approximately 5 minutes.

The second part of the test consists of cleaning the stimulated area, thoroughly drying, then collecting the sweat in a plastic coil. Thirty minutes later, the collected sweat is sent to a hospital laboratory for analysis. A minimum of 15 uL of sweat must be collected in order for the test to be valid. This is why it is so important that the patient is well hydrated.

The entire collection procedure takes approximately 1 hour for both arms.

What does the sweat test reveal?

Your physician has asked that this test be performed to rule out the presence of cystic fibrosis (CF), an inherited disorder of the lungs, intestines and sweat glands. Children and adults with CF have an increased amount of chloride in their sweat. In general, sweat chloride concentrations equal to or less than 29 mmol/L are normal (does not have CF); values between 30 to 59 mmol/L are intermediate, and sweat chloride concentrations equal to or greater than 60 mmol/L are consistent with the diagnosis of CF.

For individuals who have CF, the sweat chloride test will be positive from birth. Once a test result is positive, it is always positive. Sweat test values do not vary when individuals have colds or other temporary illnesses.

Is there any preparation for the sweat test?

There are no restrictions on activity or diet or special preparations before the test. However, please plan on coming 1 hour before you appointment to go to Mark Wallace Tower (MWT) if you need to be registered.  Please have the patient drink plenty of water and do NOT apply creams or lotions to the skin 24 hours before the test.  If the patient develops if any of the following, call 832-824-5152 and re-schedule the test.

• Acute Diarrhea
• Dehydration
• Severe Eczema on both arms
• Fever
• Vomiting
• Is placed on an open oxygen delivery system
• IV placed in both arms.

All regular medications may be continued and will have no effect on the test results.

When are sweat test results made available?

Sweat test results are usually available to your physician on the next working day after the test is performed. In a small number of cases, the quantity of sweat obtained is not sufficient to give an accurate result, and the test may need to be repeated.

Can the test results be inconclusive?

Yes. In a small number of cases, the test results fall into “borderline” range between not having CF and indicative of CF. In these situations, repeat sweat tests, as well as other diagnostic procedures, may need to be carried out. These will only be done after consultation with a physician.

More Information

• Cystic Fibrosis Foundation

References & Sources

"Sweat Testing." Cystic Fibrosis Foundation, 2017