Negative amniocentesis results - positive sweat test

Question

During my pregnancy the results showed that my baby would be a carrier (heterozygote) - 85% checked. My baby is now 7 months old and the sweat test was positive for CF? Is this possible and in which cases?

Answer

Dear friend,
You did an amniocentesis and one mutation was found in the fetus, but afterwards the sweat test was positive for CF in your baby.
The CF gene is very large and there are many mutations. Up till now there have been scanned round 1900 mutations. Even if all the CF gene is scanned, there will be a percentage of mutations not scanned.
For this reason the result of the molecular analysis cannot exclude the possibility of a mutation, but it reduces the possibility of one.
Even when both parents are checked, the possibility of the newborn having CF cannot be excluded, but this possibility is very greatly reduced. This reduction is as great as the percentage of checked mutations.
CF can be found in such cases, as you found out in your baby, something that has happened in other cases as well (rarely but it has happened).

In your case, where the percentage of mutation checked was 85%, the possibility of having a baby with CF is 1/644 births. This estimation is based on the analysis by Wilford BS & Fost N., (1990). JAMA 263(20) 2777. The Cystic Fibrosis gene: medical and social implications for heterozygotes detection.
Yours friendly,
Dr. Stavros Doudounakis

24.08.2012