Cystic Fibrosis

Question

My son had a negative CF test some years ago. He’s being retested now, could the test become positive ? Thanks for your answer, Liane Kips.

Answer

Dear Mrs. Kips,
Your question is indeed appropriate. Because cystic fibrosis is an hereditary disease, it is impossible to have no CF at the start and develop it in later life. It is thus a question of testing itself: which type of test was performed years ago, for which disease and what method ?
It is possible that for children and adults with signs and symptoms compatible with cystic fibrosis, medical doctors test for CF more than once in order to exclude or confirm the diagnosis. There are basically three types of tests for CF diagnosis: sweat test, genetic testing and complicated tests measuring the function of the salt or chloride channel. We will concentrate here on the sweat test because it is the most commonly used test. It is a good and reliable test, if performed correctly. With this test the chloride concentration in the sweat is measured, and this value is higher in persons with cystic fibrosis compared to persons without cystic fibrosis. The standard methods for performing the tests, the so-called methods according to Gibson and Cook, is the most reliable one. With this method “Pilocarpine” (a substance stimulating sweat production) is applied on the skin with the aid of the forearm with a miniscule electrical current. Sweat is then collected in a cloth or a collecting tube. The amount of sweat obtained is weighted and the concentration of chloride measured in the sweat. (Chloride is the negative ion, part of sodium chloride of kitchen salt). There are other methods for sweat testing like conductivity and osmolarity but these are less reliable.
All part of the tests have to be performed correctly in order to obtain reliable result: use of the correct method, experience of the person taking an analyzing the test, a minimum number of sweat test per year performed in a specific centre in order to have enough expertise, collecting at least 100 mg of sweat per arm. The latter may be a problem in younger children, a reason why the test may have to repeated.
The test result should report the amount of sweat, the concentration of chloride and sodium (the latter being the positive loaded chemical part of kitchen salt used as quality control). Diagnosis of cystic fibrosis can be made if the chloride concentration expressed in mmol/l or mEq/L is above 60. If the chloride concentration is below 30 mmol/l; the sweat test is normal and it is very unlikely that the person tested has CF. A test result between 30 and 60 mmol/L is considered as a grey zone: a classical form of cystic fibrosis can be excluded but milder forms of the disease may be present. In such cases additional tests are needed for more specific diagnosis: genetic tests and/or other tests measuring the functioning of the chloride channel (Nasal Potential Difference test and Intestinal Current Measurement Test). You can read more on these tests under Diagnosis and Diagnostics.
From this information it is clear that there may be several reasons to repeat the CF test. The most frequent reasons for retesting are related to the sweat test. Incorrect performance of the sweat test, insufficient sweat obtained, non specific results reporting etc. The sweat chloride result in the grey zone is also a reason to repeat the sweat test and plan additional testing. For more information concerning these more specialized tests we refer to the information on diagnosis and diagnostics.
We hope this answer clarifies your question.
Kind regards
Dr. M. Boon and Prof. Dr. K. De Boeck

28.01.2010