Different sweat test results?

Question

Hello,

I am writing concerning my four-year-old son, who was born prematurely in the 30th week of pregnancy and has had recurring infections of the bronchia ever since. He has also had pneumonia three times so far.

Until now, he has always been treated for chronic bronchitis. For further diagnostics, he had a sweat test done; the value was 84mmol.

After that, his tonsils were made smaller, since he had so-called Kiss tonsils which were very big.

We were sent to a CF clinic where he had a second sweat test done today. The result was 16mmol!!

The doctor said the CF clinic had the “safer” test since it would measure the chloride directly and that the first test was surely measured differently. We should trust the current result.

Of course we are relieved, but the pathology over the past years still leaves us uncertain, since there will be no targeted therapy again but the paediatrician will keep prescribing broad-spectrum antibiotics.

How safe would it be to have a gene test done now, and where can we do it? How can such a significant difference in the values come about?

I would appreciate a brief statement from you.

Kind regards.

Answer

Hello,

you are saying that your four-year-old son, who was born prematurely in the 30th week of pregnancy, keeps having problems with the respiratory tracts and has already had pneumonia three times. One clinic did a sweat test that yielded a value of 84mmol. At a check at a CF clinic, the value was 16mmol. You are confused due to the significant difference of the two results and are asking whether it would be better to do a gene test in this situation.

The different results are most probably caused by the fact that two different testing methods were used. Many smaller children’s hospitals use a method which indicates the chloride value indirectly via a conductivity measurement in NaCl equivalents. The machines used in this method are rather low-priced. Unfortunately, the results are rather inaccurate. It is very likely that the CF clinic does these tests by using a chloride meter to directly measure the chloride in the sweat sample. This is the so-called gold standard. In case the test was done only on one arm and you therefore have two isolated individual results, I would suggest you request a second test in the CF clinic for your reassurance. With great certainty, the result will be in the same range as the first test taken there. Then you can be completely at ease. A gene test will then be absolutely unnecessary and not justified.

Every once in a while, premature babies have problems with the respiratory tracts and repeated obstructive bronchitis as well as occasional pneumonias during the first years of their lives. This is the case especially when artificial ventilation was necessary temporarily. Perhaps this could explain your child’s problems?

All the best to you and your son.

Kind regards,
Dr. H-G. Posselt

02.05.2012