IRT value

Question

Hello!

Since our 3 year old son has CF (del-F508 mutation) our newborn baby (35th week of pregnancy) was screened more extensively. As a result we were told that the IRT value is unremarkable. We assumed, thus, that CF can be excluded. But sometime a sweat test is planned to be done. Wouldn’t a genetic test make more sense? Would we have to assume the same mutations? Since our older son already tasted very salty as a baby wouldn’t our baby have to taste as salty as well?
Many thanks for answering.

Answer

Hello,

an inconspicuous IRT value in a newborn screening excludes CF to a great extent. However, a very small probability to have CF cannot be excluded completely even if the IRT value is inconspicuous. This can be the reason why you was advised to have a sweat test done for further clarification. The sweat test (which, however, cannot be done really reliably during the first postnatal weeks) has the following advantage in comparison to the genetic testing: the result [of the sweat test] does not allow a conclusion whether a healthy child is a heterozygous genetic carrier of CF (i.e. has inherited the mutation from one parent but not from the other parent) or whether the child has inherited the non-mutated CFTR-gene from both parents. One wants to avoid to disable a healthy child from the possiblity to later decide himself/herself if he/she wants to know if he/she is a heterozygous genetic carrier or not. According to the “Gendiagnostikgesetz” [German law: Gene Diagnostics Law] a heterozygous testing is not done for healthy children. A genetic testing performed on your newborn baby would only be done if there is a suspicion of CF (e.g. borderline or pathological sweat test). If the mutation(s) of both parents of a child with CF are tested and verified, a secured statement would be possible if a further child of these parents would be concerned or if the suspected diagosis is not confirmed.

Best regards
Prof. Stuhrmann-Spangenberg

23.03.2010