Testing CF

The sweat test has been the cystic fibrosis (CF) diagnosing for more than 40 years. The test can be performed on individuals of any age. 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 or leg. An electrode is then attached to the arm or leg, which allows the technician to apply a weak electrical current to the area to stimulate sweating. The second part of the test consists of cleaning the stimulated area and collecting the sweat on a piece of filter paper or gauze or in a plastic coil. The entire collection procedure takes approximately one hour. In some states in the United States, all newborns are checked for CF with a blood test, although the blood test isn't as good at detecting the disease as the sweat test.

The sweat test can help rule out the presence of CF, an inherited disorder of the lungs, intestines and sweat glands. Children and adults with CF have an increased amount of sodium and chloride (salt) in their sweat. In general, sweat chloride concentrations below 40 mmol/L are normal (does not have CF); values between 40 to 60 mmol/L are borderline, and concentrations 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 change from positive to negative or negative to positive as a person grows older. Sweat test values also do not vary when individuals have colds or other temporary illnesses.

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