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Diagnosis respectively exclusion of CF at adult age

Question
Dear ladies and gentlemen,

I suffer since 2 years without cessation from strong cough with sputum (I am non-smoker), burning and tightness in the throat, hoarseness, cramps in the thoracic cage, back pain, a feeling of pressure in the upper abdominal tract, frequent diarrhea. The cough starts in the mornings, about 10 minutes after getting up. Altogether I cough in the course of the day about 3-4(!) hours, sometimes even more, with cough-free intervals inbetween. At night I am mostly free of coughing.

To the diagnostics so far (I try to make it short): 2 gastroscopies, 2 thorax x-rays of the lung, 2 CT scans of the lung, allergy test, investigations of blood, 2 bronchoscopies, CT scan of the sinuses, presentation at the rheumatologist (all blood values in the normal range except for an increased ANA value, the lower values within the normal range), MRI of the cervical spine and the thoracial spine, ultrasound of the thyroid gland, lung function test, standard blood values - all that within the past 2 years and everything in the normal range.
An underlying reflux/silent reflux as cause of the cough could be excluded by a 24-hours pH measurement/ impendance. Furthermore, a probatoric therapy with a blocker of the gastric acid did not bring improvement.
I should undergo (according to my own wish) a sweat test, in order to diagnose respectively exclude CF as a possible cause for the permanent cough with sputum. As I am already 46 years old and have suffered only from recurrent and strong rhinitis in my former life (before the permanent cough) so far, in case of a positive result it would be, according to my understanding, an atypical form of CF.

Now to my question:
Which kind of diganostics would make sense in my case? Would a sweat test be more sensible or a genetic investigation or probably a measurement per Ussing-chamber?
Many thanks for your efforts and best regards,
Answer
Dear questioner,
in the diagnostic to exclude or verify a CF, the performance of the sweat test is the gold-standard. In here it is important, that the test is done two times at a certified CF Center. After this, further diagnositc steps should be taken according to the result. All results should be brought together and judged by an experienced pneumologist, who also knows you in person. If necessary, the pneumologist will get in contact with a CF Center. The judgement of the results without knowing the patient, is not possible via the Internet. After the sweat test, it should be decided, if a measurement of the nasal potential difference (NPD) and/or an intestinal current measurement (ICM, Ussing-chamber) and/or a genetic investigation is necessary.
For your physician in charge we point out the guideline for the diagnostic of CF. It deals here with an "S2-consensus guideline about the "diagnosis of CF" (AWMF 026-023; German guideline) under the leadership of the "society for pediatric pulmonology" latest update 06/2013. On page 20 of the guidelines there is an algorithm on the procedure in case of the clinical suspicion of CF.
In the comment on the algorithm it is written: "In case of clinical suspicion of CF the sweat test is in the first place of the diagnostic algorithm. In case of a sweat chloride greater or equal 30 mmol/l or a sweat conductivity greater or equal 50 mmol/l, respectively an unsuccessful performance of the sweat test or a positive newborn screening (if available) further diagnostic steps should be done at a CF center. In case the sweat chloride value should be between 30-59 mmol/l, a CFTR genetic investigation, an NPD and/or an ICM are necessary. The investigation for the most frequent CF-causing CFTR mutations should be the next following investigation due to the good availability and the quick results. In exceptional cases a complete investigation of the CFTR gene could be sensible even without a preceeding investigation of the most frequent mutations, if otherwise a very low dection rate could be expected due to the ethnic background of the patient. In case of a missing confirmation of the diagnosis, the next steps would be a complete CFTR analysis and the electro-physiological investigations NPD an/or ICM. The order of the performance of the investigations depends on the availability of the investigations. A complete CFTR investigation should also be done in patients with a normal electro-physiology, as in single cases, patients have been described with two illness causing mutations in spite of a normal electrophysiology. On the contrary, patients, whose complete CFTR analysis did not show two illness-causing mutations, should undergo an electrophysiological investigation. Concerning the algorithm on CFTR-associated diseases we point out the detailed algorithms in the European consensus document".
I hope to have helped you with my remarks and stay with my best regards,
Yours
Dr. Christina Smaczny
24.01.2015