Sweat test

Question

Dear expert team,

Our 4-year-old daughter has recurring obstructive bronchitis since three years. We were in the children’s hospital twice for having a sweat test done. The values were on the borderline (41 and 51 Na). The test that was done in the outpatient department was negative (16 Chloride). Can Cystic Fibrosis be excluded for sure? How can such differences of sweat test results be explained?
Elastase detection was negative while she had never any problems with bowel movement!
Many thanks for giving your opinion!

Answer

Hello,

You report about your 4-year-old daughter who underwent two sweat tests with a value of 41 and 51 mmol/l of sodium (Na) because Cystic Fibrosis was suspected. You also report that a control was done at a CF outpatient department and the value was 16 mmol/l of chloride. The detected elastase value was normal [this is how our expert understood “negative” in the question].
You ask how such different values can be explained and if CF can be excluded for sure.
First please note that there are pretty different methods for doing a sweat test. In some hospitals still the sodium level (Na) is detected although there is international consensus that the chloride level should be detected. Furthermore, sweat tests in some hospitals are not always done by trained staff so that seldom false-positive or false-negative findings are collected.
It can be assumed that in the CF outpatient department the chloride concentration of the collected sweat was measured by “pilocarpine ionotophoresis”. In this case, the value should be reliable. If you want to be absolutely sure you could ask for a control test for the value of 16 mmol/l in the CF outpatient department.

Best regards,
Dr. H.-G. Posselt

16.06.2015