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Cystic Fibrosis

Can a sweat test be wrong?



My son, who is 13, scored a 49 on his first sweat test and a 61 on his 2nd. He is very healthy, the tallest kid in his grade and is very physically active. He was referred for a sweat test by his ENT Dr - because of an upper respiratory infection. (This is a routine referral, our ENT refers all children for this test). My son also drinks tons of Gatorade. He drank two bottles of Gatorade the night before his 2nd test and a half a bottle on his way to take the test. Could that have affected the sweat test results? We are scheduled to see a pulmonary specialist on July 10th, but I am going crazy waiting...any information would be greatly appreciated!


The short answer is that sweat tests need to be done at a certified Cystic Fibrosis Foundation lab, or they are not necessarily reliable. If it was done at such a lab, the results are likely correct, but the interpretation is where the question is.

The first, 49, is in the grey zone, with 61 being positive. These results should not have been affected by the Gatorade. Most people with tests in this range do have some abnormality in how the lining of the lungs and sinuses work, and many do have CF, but it will be necessary for the doctors and other health professionals at the CF center where he will be seen to obtain genetic testing and look at his symptoms as a whole to help decide if he does have CF.

Those with borderline sweat tests who do have CF are likely, but not certain, to have milder CF than those with more strongly positive tests. We follow a number of people at our center about whom we are not certain, just to keep an eye on them, seeing them at a time separate from our CF patients.

It is also important that your son be evaluated by someone at a certified CF center to help in diagnosis, as it can be more art than science, sometimes, and requires a level of expertise beyond that of most general pulmonologists.

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Response by:

John S Heintz, MD John S Heintz, MD
Clinical Assistant Professor of Pediatrics
College of Medicine
The Ohio State University