How confident is the genetic testing

Question

Hello,
I have a 13-year-old daughter. The sweat test had been pathologic according to the report from the hospital. She has had a value of 66.
After that, a genetic testing had been performed. This was negative.
How confident is it that she does not suffer from CF.
Best regards,

Answer

Hello,
the information we have about your daughter is very limited. You report, that a sweat test had been performed to your daughter and the result has had a value of 66 (mmol/l for chloride?). Then a genetic testing had been performed and it had been "negative". Now you ask, how certain it is that your daughter does not suffer from CF.
To the result of the sweat test, the following remarks can be made. A repeatedly detected value of the swea test of over 60 mmol/l chloride proves an underlying CF, in case there are clinical signs of a CF. A prerequisite is however, that the sweat test had been performed in an experienced laboratory on this field. The value of 66 mmol/ l is just over the borderline, and therefore always a control investigation should be performed that confirms this borderline value, as it happens again and again that a false positive result occurs due to a laboratory error. In case the result could be confirmed with a control investigation, one can assume the following situation. The borderline value of 66 suggests that with a certain probability it deals not with the full picture of CF, for example only lung problems exist and the pancreatic function of your daughter is normal.
To the result of the genetic testing the following can be said. We judge your statement that the test had been negative as a hint that no CF typical mutations have been found with the test. With the actual ususal test procedures 29 up to a maximum of 36 of the most frequent mid-European CF mutations of the CF-gene are investigated. In patients with only a slightly increased sweat test value there is often at least one "more seldom" mutation underlying, that has not been detected by the test. In rare cases, there are two "seldom" mutations underlying in case of a CF. Therefore, the gained result of the genetic testing of your daughter does not exclude a CF for sure.
In summary it has to be said, that your daughter should be presented at a specialized CF center in order to further clarify the situation. As a first step, the sweat test should be repeated (if possible as a double measurement at both arms). In case the result should be normal, your daughter is free of the suspicion of CF. In case the result should be comfirmed, the physicians of the center will discuss the further procedures with you.
Yours sincerely,
Dr. H.-G. Posselt

02.12.2013