CF is sustected - test probably false-positive?

Question

Hello,
our daughter (3 years) has been in hospital since last monday with a severe pneumonia.
As the inflammation did not disappear in spite of intensive treatment with antibiotics, a test for CF had been done last monday, too. The result of the sweat test was quite a shock for us because the result was 91 mmol/l.
In the first line this result did not mean a thing to us and it was a bit unfortunate as our daughter had been discharged about 2 hours after the result of the test. They only told us that because of that test the suspicion of CF became more likely and that we should come again in 2 weeks in order to carry out a second test.
With such a result and without any further information being sent home is of course quite questionable.
But to my question: on the one hand we are "used to" some fatal diagnosis concerning our daughter as she had been a preterm baby with severe and life-threatening complications after birth. But on the other hand the result of the test does not make any sense to us, as our daughter does not really show any symptoms, which would be typical for CF (it is my opinion, at the moment our information about this illness is quite poor).
When suffering from the pneumonia she was able to expectorate the mucus well without any specific drugs and otherwise she is healthy except the latest pneumonia and has nearly any problems with striking cough.
However, she has had a double side pneumothorax after birth, but even because of this she does not have any special complications today.
How is the probability that there are sources of error with the result of 91 mmol/l and where are the limits? We have read something about 40, 60 or 90, but do not know what is right however.
Is it for example possible that administration of Sodiumchloride intravenously during the stay can influence the result of the test? Additionally, she receives Movicol regularly for about 8 weeks due to the lethargy of the bowel. Is that treatment possibly a source of error that could lead to a wrong positive result of the test? Futhermore I have heard that if the amount of sweat during the test is too small this could lead to mistakes, is that right?
Up to now we did not notice that her sweat was salty or something like that, even in the summer she only hardly perspirates.
I would be glad to hear your opinion about that.

Answer

Dear parents,
I can well understand that you are shocked by the result of the sweat test and that you feel left alone with the uncertainty whether your daughter suffers from CF or not. It it good that the doctors of the hospital had the illness of CF in mind because of this heavy pneumonia and therefore started the relevant diagnostic procedures. Especially with a premature child many pulmonary problems are thought to be due to the prematurity and other causes are not considered so easily.
The sweat test is the first diagnostic step in order to diagnose CF. You are completely right in assuming that there might also be wrongly positive results by using this method.
One reason for these wrongly positive results is that the standard method (pilocarpine ionotophoresis) is not used and another reason is that there are differences in the performance of the test, e.g. if the quantity of sweat is too small (the aim is 100 µg). I cannot tell you how high the probability of the source of error exactly is.
At the moment 40 mmol/l chloride is considered to be normal, 40-60 mmol/l is at the limit, more than 60 mmol/l is abnormal. These results refer to the so-called "typical CF". There are also types of CF where the result of the sweat test is normal, though this case is rather seldom.
In case of your daugther the first sweat test showed an abnormal result. Therefore the next step is the performance of a second sweat test in order to exclude a wrongly positive result. The first sweat test was carried out during a heavy pneumonia. Of course an exsiccosis, for example, can make the result of the test wrongly positive. An exsiccosis means that your daughter didn't have enough liquid because of the pneumonia (as she didn't drink and eat enough and therefore probably got sodium chloride intravenously).
It is unlikely, that the results of the test are affected by the therapy with Movicol. In order to exclude that the results of the test are affected by an infection, by changes of the fluid balance, by fever etc., the second sweat test is carried out in the so-called infection-free interval, that means in your case after two weeks.
If this result is abnormal again, this confirms the diagnosis of CF. A genetic analysis can then be useful in order to know the exact mutation.
Concerning your question if it is possible that your daugther suffers from Cf though she doesn't show any specific symptoms, I must definitely answer that it might be possible. The course of this illness can be quite different: with or without problems of digestion, with early pulmonary symptoms or with pulmonary symptoms that occur later or even sometimes only at adult age. An increased concentration of sodium chloride in the sweat cannot always be tasted, though this phenomenon is mentioned in all books and reports.
Not all patients show an increased perspiration. Only the next diagnostic steps will clarify the situation.
This time of uncertainty is certainly very difficult. I hope that the result of the diagnosis will be normal and I wish you all the best.

Yours sincerely,
Silke van Konigsbruggen

28.04.2008