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Sweat test and diagnostic algorithm

Dear expert team,
I have read the legal disclaimer and hope my question is possible. I do not want to get a diagnosis of course, however I am looking for further, additional expert opinions.
Our son, 6 years old, is at the moment investigated for CF. This was because I had been on rehabilitation due to my own lung disease (LAM, that has nothing to do with CF, other mutation etc.). During rehabilitation I got in contact with CF patients and told the story of my son, who has like me frequently infections of the airways. During the discussion it was speculated if those infections could have something to do with my lung disease and I reported the suspicion of my physicians, that I indeed would also have a so-called slight infection-asthma. In my family there is strong allergic rhinitis (my father and my brother) and my son is also a “cougher”. With that I mean: colds are always starting in the nose, are going down and turn to the bronchi. There the cough is lose however is long-lasting, sometimes for 2-3 weeks. The cough is therefore productive, rarely my son is obstructive and a real bronchitis or even a pneumonia has never occurred. He never got an antibiotic, as it was not necessary.
However, my son is sweating easily and is underweight (18,5kg) with a normal height (118cm). He does not eat much, however healthy food, much fruit and salad. He is very sportive and sweats easily, he is however not out of breath.
Due tot he talk with my CF friends I contacted the pediatrician, who made an appointment for doing a sweat test in the near children’s hospital. The first chloride value was unfortunately at 50. I have been told, that this could be due to a lack of fluid. My son is not drinking much. The following two days I let him drink quite much (like it was possible) and the second sweat test value was then at 31.
Then we had a stool sample investigated. The pancreatic elastase had a super value (I do not have the exact values), fat in the stool was slightly increased. Next step: CF center. Here a genetic testing is done and a third sweat test.
I could just wait for the values. However I cannot sleep anymore. After my own diagnosis of a very! seldom lung disease that is life shortening out of total health in February this year, now this nightmare. I often think “if I never started it…” I feel like in a diagnostic mill, a machine that goes on its way. It is good, that one looks so precisely! However it is a borderline value after a borderline value and the genetic test is investigating the most frequent 30 mutations out of 2000. If the test would be negative, which I hope very much, it is going on….according to the diagnostic algorithm. This is bringing me down.
My question: how do you judge the situation, is the suspicion really realistic? What else could be the reason for an increased chloride value? What is the genetic testing for that does not test for 1970 other mutations, if it is negative? How long does such a test last?
Many thanks in advance and please do excuse that this was quite a long text and that it contains several questions.
Best regards,
[name of mother was taken away by ECORN-CF]
as your report and your questions are quite extensive, I will only answer it in summary. You report, that your 6-year-old son has on the one hand quite often infections of the airways, that last for 2-3 weeks. Your son however has never taken antibiotics. He would be quite thin. Because of the contact to CF patients during rehabilitiation, you were considering doing a sweat test to him.
During the first years of life, about 6 infections of the airways per year are quite normal for children. The fact, that he never needed an antibiotics, is very calming. You report, that the function of the pancreas has been tested and that it would habe been normal. The two sweat tests, that had been done, had one time a value of 50 mmol/l and the second time a value of 31 mmol/l chloride. Now you have been proposed to have a molecular genetic investigation done.
The step-by-step procedure according to the algorithm is right. Depending on the laboratory the proposed genetic diagnostic takes 7-14 days. This is of course a very burdening waiting time, that you are experiencing now for a longer time period. However, there are arguments against an underlying CF.
1) Until now, your son did not have a severe infection of the airways.
2) The function of the pancreas is normal.
3) The results of the sweats tests have been quite different. The clear difference of the values is not usual. However, one value is in the normal range and probably the third value will also be normal. If the genetic investigation will not show any mutation, it seems very unprobable to me that CF is underlying and I would recommend not doing further diganostics.
In case a mutation should be found, it would make sense to have a cell-physiologic investigation of a sample of the rectal mucosa done, in order to have absolute security.
Overall, I do not think your son suffers from CF and I hope, that this estimation can be confirmed rapidly.
Best regards,
Dr. H.-G. Posselt