5-year-old daughter, sweat test elevated?

Question

Hello,
I have been with my 5-year-old daughter yesterday to the sweat test, because she is permanently ill. She has been ill this winter 7 times in 3 months always with over 40 fever.
The sweat test showed a result of 48 mmol/l. The nurse in the hospital said that everything was o.k. and we need not to come anymore.
However, now I have read before the test, that values above 30-40 mmol/l have to be controlled.
Now I am concerned, as in the doctors' letter totally different reference values are mentioned. There the borderline region is only above 60 mmol/l and certainly positive would be above 90 mmol/l.
Can the reference values vary or is it due to different measuring devices?

Answer

Dear parents,
you report that your daughter has been ill with an infection with fever very often last winter. This can be totally normal at that age.
A sweat test is performed in order to find out, if a child suffers from Cystic Fibrosis. CF is a chronic disease of the lung in children and adults (about 1:3000), which can be accompanied by few symptoms, like e.g. clearing one's throat, coughing slightly but also with more severe symptoms like e.g. productive, chronic cough, shortness of breath up to recurrent infections of the lungs.
In the sweat test, the SodiumChloride concentration of the sweat, which is increased in that illness, is measured. Hereby it is important, that a standardized measurement is conducted: a pilocarpine-ionotophoresis with the measuring of the chloride and a sufficient amount of sweat, a sole sodium measurement is done additionally as quality control and should only vary slightly from the chloride value (under 20mmol/l; otherwise the sweat test should be repeated). A conductivity measurement is not acceptable for making the diagnosis and should only be performed as screening test.
You report a sweat test value of 48 mmol/l; is this the sole chloride value? Or the SodiumChloride value with a small amount of sweat? In order to be able to interprete the test correctly, information about the amount of sweat, the chloride and sodium value are needed, as well as the method, which has been used. The normal ranges mentioned in your doctors' letter indicate more a conductivity measurement; with this test values over 80 mmol/l are pathologic, values between 50-80 mmol/l represent the borderline region.
At a result of 48 mmol/l I would recommend a measurement according to the accepted measuring standard (pilocarpine ionotophoresis with chloride measurement), not a conductivity measurment.
If we assume a pilocarpine ionotophoresis with a sufficient amount of sweat and a sole chloride value of 48 mmol/l, the value is without doubt in the borderline area.
Chloride values over 60 mmol/l are named pathologic, that means a CF is very likely, values between 40 and 60 mmol/l are borderline, according to new assessments even between 30 and 60 mmol/l; under 30 mmol/l the typical form of CF can be mostly excluded. However, a normal sweat test does not exclude generally a CF; there are forms of the illness, which can be accompanied by a normal sweat test.
Best regards and much luck,
Dr. S. van Koningsbruggen

18.06.2009