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CT of the lung appropriate?

I am 35 years old and in the frame of an genetic investigation it came out that I am a carrier of CF. When I was a baby, I had a strong pneumonia. I suffer from allergic asthma and have slight clubbing of the fingers. A recently performed sweat test had a result of 42 (control area).
In the department of pulmonology (of a German university hospital) a lung function test had been performed. The result was without pathological findings (98%). The blood values had no pathological findings, either. Now, an ultrasound of the upper abdomen and a lung CT are planned just to be on the safe side. My question: is it not overdone to expose oneself to radiation, if I have otherwise no complaints? Or do you recommend a CT scan in this situation?
you want to know with your question, if in your situation as an asthma patient, the performance of a CT scan of the thorax is necessary to exclude the diagnosis of CF. In order to be able to answer your question satisfyingly however, detailed information is still missing, like e.g.:
- why a genetic test had been performed at all, if there were otherwise no hints for CF, you are doing well and you do not have any complaints or other pathological findings?
- how extended the genetic test was? (how many mutations have been tested?)
- which electrolyte is meant with the sweat test (chloride? sodium?)
- which lung function parameter is described by you with 98%?
- which blood values are you talking about?
- what did the physician motivate to think about CF in your case?

As I do not have an answer on these questions, I will try to formulate an estimation on the basis of your information:
- a CF-carrier is healthy (genetic finding) and not suffering from CF, therefore no indication of CF here
- a chloride content of the sweat of 42 mmol/l is a "negative" value, respectively is just above the borderline of 40 mmol/l to the so-called "intermediate range", that is not clearly positive nor negative, therefore also here no confimation of CF
- however, a normal lung function, normal blood values (which?), no other complaints (digestive complaints?) do not rule out CF, if there would be other hints on it
- clubbing of the fingers occur in general with certain illnesses, however in rare cases, these "untypical" clubbing of the fingers can also be inherited and is not a symptom of an illness
- in case further investigations concerning a possible lung disease are present, then a CT scan of the thorax is recommendable (only a physician who knows you and all of your results can make a decision on this)
- the diagnosis of an "atypical" CF in case of lacking confirming genetics and a normal sweat test, but other indications of the illness are present, can also be made via an NPD (nasal potential difference) measurment and/or an ICM (intestinal current measurement). Both investigations however are only done in the frame of scientific questions and not covered by the insurances.
You can see from my remarks that a clear answer on your question is not possible via the internet, but I hope to have helped you a little furhter with the given information.

Yours sincerely,
Dr. Christina Smaczny