False positive sweat test

Question

My son has had four suspicious sweat tests (last 96). The simple gene test was negative. He has to go to a specialist clinic now to have another gene test done. My question: how is he to have cystic fibrosis after all?
Here still some information and answers....
My son is 12 years old.
He has been investigated about 4 years ago due to his height. In the frame of these investigations, a sweat test had been perfomed automatically. As this test had been positive, again 2 or 3 tests have been performed later. All these have been highly positive. Then a genetic test had been performed with the 30 most frequent mutations and the test was negative. We have then been told by the chief physician that everything would be o.k. and we would not have to undertake anything.
So, because he was badly off lately and he had to go to hospital more frequently (due to strong abdominal pain and diarrhea) another sweat test had been performed, which had, as described before, the result of 96. Now he should be investigated in Bochum, Germany.
What speaks for this disease is: he is very small (12 years old, 135 cm high and 28.5 kg), permanent diarrhea (since his early childhood), probably his exercise-induced asthma and he has a fructose-intolerance for some weeks.
Many thanks
Yours sincerely,

Answer

Hello,

you write that your son has had four sweat tests which all returned suspiciously high results, with the last test yielding a value of 96. A subsequent “simple” gene test was negative.

First of all, the question is whether the sweat tests were carried out at a specialized CF centre. If so, one can assume that the results are reliable. If the test was done in another laboratory or practice, it would be worth considering repeating the sweat tests at a specialized centre first.

The currently common gene tests are used to check for about 30 of the most common mutations of the CF gene prevalent in the central-European population. With these mutations, more than 95% of CF patients are determined. It is very rare a CF patient with a sweat test result as high as your son’s is not tested positive for a mutation. In the situation at hand, it is, of course, also important to know which troubles of your son prompted you to take the sweat tests.

So the following steps should be taken:

1) In case your son has not been treated at a specialized CF centre, you should immediately consult one.

2) If your son is already being treated at such a centre, a so-called potential difference measurement of the nasal and rectal mucus membranes should be considered initially, since this test will certainly answer the question whether your son does have CF or not. An extended gene test, i.e. a sequencing of the entire CF gene, may have a certain chance of clarification in case your son does indeed have CF. However, this test is extremely expensive (ca. 8.000 €). As fate may have it, only one mutation will be found, and then the potential difference measurement has to be done anyway. Therefore, the potential difference measurement should be done as a next step.

With 12 years and a height of 135cm your son lies under the normal range of children of the same age. His body weight of 28.5 kg however goes well together with his actual height. In case the small body height should be due to CF, your son is supposed to have a hypofunction of the pancreas. This hypofunction can be diagnosed easily via an investigation of the Elastase I in the stool. In case his pancreas works normally, it is highly unlikely that your son is so small because of CF. You report that your son is going to be investigated in the university hospital of Bochum, Germany. Here you are at the right address. Prof. Ballmann is leading the CF center there. He will for sure initiate the necessary and right investigations.

I wish you and your son much luck and that the actual insecurity can be elilminated fast.
Yours sincerely,
Dr. H.-G. Posselt

30.08.2011