Suspicion of CF

Question

Hello, a sweat test had been done to my child (10 years). The result was 61. The doctor did now perform a test of the blood, which does however not point at CF, but he wants to redo the sweat test in 2 weeks. He has asked several questions, which we acutally can answer negatively, e.g. if there are any problems with the weight or thriving.
Now I am a bit confused, can the special blood test exclude CF for a 100%?
Many thanks for your answer

Answer

Dear questioner,
a specific blood test looking for genetic changes in the "CF-gene" can exclude CF with high security (means near to a 100%), however, a negative result can not give a full 100% security, as it covers not all possible changes which is explained below.
Such a test investigates in the CF-gene those changes, which lead to the illness. In case one finds 2 changes, one can assume that the patient suffers from CF. The investigation however is very expensive and should therefore be done only, if there is a really hard suspicion. Such a suspicion we have in case the patient shows the typical symptoms of CF and the so-called sweat-test had shown in 2 investigations a pathologic result. Sometimes the sweat test is conducted with the help of the conductance. Here we have more often high and therefore pathologic results, which can not be found after control with the standard method.
In your son the value of 61 has been detected, whereas you did not write with which method the sweat test had been conducted. In case the value should have been measured with the conductance method, another testing of the sweat electrolytes (sodium and chloride) should be done at a specialized CF center. In case with this method there should also be an increased value, the sweat test should be repeated and in case the values are increased again, a detailed blood investigation should be performed.
As there are more than 1600 different changes, which can be detected in the blood test, one has developed for saving money investigation sets, with whom the 30 most frequent changes can be detected. In most cases one can find in middle Europe with this test the illness-causing changes. I think, that this test had been done to your son. Only seldom it is therefore necessary, to widen the test to the other 1570 changes.
For your son this means, that a second sweat test should be performed in any case, however with the detection of sodium and chloride. Will the result be pathologic, one should do an extended investigation of the blood.
Yours sincerely,
DR. E. Rietschel

09.11.2009