Sweat test 3 times borderline

Question

Hello,
my son, 3-years-old, has had a sweat test for 3 times. Always in the borderline zone. Now a genetic investigation is done. The physician said, however, I should not get upset, this would not have any meaning, yet. Why can these tests be in the borderline zone and there is no illness underlying?
Thank you

Answer

Dear questioner,
results of sweat tests, that are in the so-called "borderline zone", have absolutely to be controlled with other tests, as in this case the sweat test is not sufficient to diagnose or to exclude the illness of CF. It is especially difficult in children, who appear to be completely healthy, who have no clinical complaints, and who do not show any other hints suspicious for CF and additionally have sweat test results in the borderline area. The name "borderline area" suggests already by itself, that it is not convincing enough. This can be due to many known reasons, however also due to many unknown reasons. The listing of those reasons, however, does not answer your question and above all, does not take away your worries.
After the investiagtion of the sweat test, it has always to be decided if other investigations are necessary: a genetic investigation, a nasal potential difference measurement (NPD) and/or an intestinal current measurement (ICM, in a Ussing-chamber). Therefore it is also in the case of your son of utmost importance, to have a genetic investigation done and to wait for the result.
In connection with your request, I can point to the guidelines for the diagnosis of CF. This is a S2-consensus guideline "Diagnosis of CF" (German AWMF 026-023; in German) under the leadership of the society for pediatric pulmonology dated 06/2013. On page 20 of the guideline there is an algorithm for the procedure in case of clinical suspicion for CF.
In the comment on the algorithm it can be read:
"In case of clinical suspicion of CF, the sweat test is the first line of the diagnostic algorithm. In case of sweat chloride ≥ 30 mmol/l or a sweat conductivity ≥ 50 mmol/, respectively in case the sweat test could not be performed successfully or in case of - if available - positive newborn screening, further diagnostics should be done at a CF center.
In case the sweat chloride value is between 30-59 mmol/l, a CFTR genetic investigation, a NPD and/or an ICM are necessary. The investigation of the most frequent CFTR mutations should be done as the next following investigation due to the good availability and the fast results obtained. In exceptional cases a complete investiagtion of the CFTR gene can be sensible without a preceeding investigation of the most frequent mutations, if otherwise due to the ethnic origin of the patient the detection rate is expected to be very low. In case of lacking confirmation of the diagnosis the next steps would be the CFTR complete analysis and the electro-physiological investiagtions NPD and/or ICM. The order of performance depend on the availability of the methods. A complete analysis of the CFTR gene should also be done in case of patients with a normal electrophysiological investiagtion, as in single cases patients have been described with two illness causing mutations in spite of a normal electrophysiological investiagtion. Conversely, patients, who did not show two illness cauing muations in the complete analysis of the CFTR gene, should undergo electrophysiological investiagtions. For the algorithm in case of CFTR-associated diseases we point to the differentiating algorithms in the Euroepan Consensus document".

I hope to have helped you a bit with my remarks and stay with my best regards,
Yours sincerely,
Dr. med. Christina Smaczny

17.04.2015