The Sweat Test

Overview

The sweat test has been the “gold standard” for diagnosing cystic fibrosis (CF) for more than 50 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 at a Cystic Fibrosis Foundation-accredited care center where strict guidelines are followed and monitored by the center director to make sure the results are accurate.

The sweat test can be done on people of any age. However, some infants may not make enough sweat to do the test. If an infant does not produce enough sweat the first time, the test should be repeated.

On this page:


What happens during a sweat test?

The sweat test measures the amount of chloride in the sweat. There are no needles involved in this test.

In the first part, a colorless, odorless chemical, that causes sweating, is put on a small area on an arm or leg. An electrode is then put over that spot. This lets the technician apply a weak electrical current to the area to cause sweating. A person may feel tingling in the area, or a feeling of warmth. This part of the test lasts about five minutes.

The second part of the test consists of cleaning the area and collecting the sweat on a piece of filter paper or gauze or in a plastic coil. Thirty minutes later, the collected sweat is sent to a hospital laboratory to measure how much chloride is in the sweat.

This sweat test takes about an hour, but it may take longer. When you schedule the test, ask how long it will take and when will you be told the results.

Click here to watch a video that further explains the sweat test and shows how it is done.

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What does the sweat test show?

Your doctor has asked that this test be done to rule out CF. Cystic fibrosis is a life-threatening genetic disease that causes mucus to build up and clog some of the organs in the body, mainly the lungs and pancreas.

People with CF have more chloride (salt) in their sweat than someone who does not have CF.

For people with CF, the sweat chloride test will be positive shortly after birth. A baby has to sweat enough to do the test. For full-term babies, this is usually by 2 weeks of age. The test is usually done between 2 and 4 weeks of age for babies who had a positive newborn screen.

Once a sweat test is done correctly and the result is positive, e.g. a high chloride level, it is always positive.

Sweat test chloride values do not change from positive to negative or negative to positive, as a person grows older. Sweat test values also do not vary when a person has a cold or other brief illness.

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What do I need to do to get my child or myself ready for the sweat test?

There is no activity limit or special diet needed before the sweat test. However, you should not apply creams or lotions to the skin 24 hours before the test. All regular medications may be continued. These will have no effect on the test results.

Babies should be fed their usual amount and at their usual times.

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When will I know the results of the sweat test?

Sweat test results are often talked about with parents at the CF care center shortly after the sweat test is done and the amount of chloride in the sweat is measured.

In other cases, the sweat test results are sent to your doctor on the next working day after the test is done. In a small number of cases, the amount of sweat collected is not enough for an accurate result. When this happens the test will need to be repeated. This is called a "quantity not sufficient," or QNS, test result.

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What is a positive result?

For infants up to and including 6 months of age, a chloride level of:

  • Equal to or less than 29 mmol/L = CF is very unlikely
  • 30 - 59 mmol/L = intermediate means that CF is possible
  • Greater than or equal to 60 mmol/L = CF is likely to be diagnosed

For people older than 6 months of age, a chloride level of:

  • Equal to or less than 39 mmol/L = CF is very unlikely
  • 40 - 59 mmol/L = intermediate means that CF is possible
  • Greater than or equal to 60 mmol/L = CF is likely to be diagnosed

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Can the test results be uncertain or borderline?

In a small number of cases, the sweat chloride test results fall into “intermediate,” “borderline” or uncertain range. This means that it is between not having CF and having CF. When this happens, the sweat test is usually repeated.

Also, other tests such as genetic testing may be done. These will only be done after you talk with your doctor.

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Where can I learn more?

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Updated 3/22/2011