Sweat test versus newborn screening

Question

Helllo,
our daughter has had a newborn screening at birth, 2.5 years ago, the result was normal.
The sweat test last week was at 95. Which test can we trust more? Other hints are cough (without mucus) and she gets infections very easily. The weight is normal.

Answer

Hello,
you report, that the newborn screening of your daughter 2.5 years ago has had a negative result for CF and that now a sweat test has had a result of 95, I assume 95 mmol/l chloride concentration. You ask, which value can be trusted.
It can be said about both test results, that a single test alone does not give sufficient security in relation to the severity of the illness. Furthermore, it has to be said about the screening, that used to be performed 2.5 years ago, that patients with CF, who were pancreatic sufficient at birth, therefore have a normal pancreatic function, were not recognized as CF patients.
This former done screening was on the basis of an analysis of the Trypsin-IRT and therefore could only recognize patients, that had a severe impairment of the pancreatic function at birth. About 20% of the CF patients however, have a normal function of the pancreas and therefore false negative test results. This could mean, that your child has not been recognized as a CF patient, because at that timepoint, the function of the pancreas was still normal.
Of course it is also possible, that the sweat test is false positive. This is especially the case, if the test was not done at a certified CF center, as not in all hospitals and offices the optimal method for investigation is used.
In any case, a once positive result has to be confirmed via another double measurement at both arms, before the diagnosis of CF can be made for sure.
If this has not been done already, please insist to have this done.
I wish you and your child much luck.
Best regards,
Dr. H.-G. Posselt

28.04.2017