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Is a single test certain?

I am 15 years old and about 6,7 years ago I underwent a sweat test for CF and the result was negative. Since some weeks however, I have possible sings of CF occurring with increased frequency. Now I would like to know if a single test can give a certain and irrevocable result...?
Certainly you are right not to trust a single negative sweat test. Making mistakes is even possible in the carrying out of a sweat test, so that it would be advisable to carry out a second sweat test, to make the possibility of suffering from CF unlikely. However, there are patients, who are suffering from CF in spite of several negative sweat tests. In case a doctor who knows a lot about the diagnosis of CF, still believes that the patient is suffering from CF in spite of negative sweat tests, he can arrange an additional test and/or a moleculargenetic investigation, which can be done from a few mililiters of blood.
With that test 85% of all underlying genetic changes of the illness can be determined to a reasonable price. If you want 100% certainity, you have to determine all possible genetic changes - up to now 1500 are known - which is only rarely necessary and unreasonably expensive (some thousands Euros).
Alternatively, and that is much less expensive, you can make the diagnosis of CF certain by carrying out a little painless biopsy of mucosa from the rectum.

In case the molecular analysis can not clarify the situation, there are some more diagnostic tools (so-called potential difference of the nasal mucosa, the mentioned rectum biopsy, analysis of the function of the pancreas (stool-fat, the pancreatic enzyme elastase in the stool), microbiological investigation of the airways, clinical symptoms).

Therefore I would advise in your case to ask your family doctor to arrange a second sweat test and than present yourself in a CF-centre. It can then be decided there if the signs that you noticed justify a moleculargenetic investigation or a rectum biopsy.

Best regards,

Dr. Ernst Rietschel