Prognosis after rectal suction biopsy

Question

Thank you for answering my previous question about the diagnosis of cystic fibrosis. Meanwhile a rectal suction biopsy was done. Also this did not bring a clear result. The amount of CFTR protein was low but there was some residual function said our pediatrician who got this answer from the CF doctor. The doctor says it’s either no CF or a very mild form of CF that my daughter can live with until the age of 70 or 75. Now everywhere I read that even mild forms of CF can have a severe course. To what degree can a doctor state, after result of rectal suction biopsy, that it is a mild form of CF and that the prognosis is always really good? Could a rectum biopsy taken in 20 years time give a different result or will the CFTR protein always have a considerable residual function? I really cannot find a lot of information on this. My pediatrician couldn’t answer me at all. She says she is not sufficiently specialized. Also I wonder how it is possible that a rectum biopsy would yield an abnormal result if no second gene mutation is found. Can being a carrier result in an abnormal rectal biopsy result?

Answer

We understand your concern but want to stress again that Ecorn should not be used as a consultation for unsolved problems. For all specific details about your child we refer you to your doctor. This forum only gives general information about the diagnosis of CF and mild forms of CF.

To draw a clear line between mild CF and certainly no CF is difficult. To make a diagnosis of CF several methods are being used but in a few cases all test will yield an unclear result and some uncertainty continues to be there: sweat analysis, CFTR mutation analysis, rectal suction biopsy or nasal potential difference test. In these cases one should look at the symptoms of the patient and the results of all tests combined. In any case the most important point will be a careful follow-up of the patient. Indeed, if CF-like disease symptoms occur in such a patient they can be treated timely. It would thus be very unlikely that somebody with a mild form of CF and a strict follow up will have a bad disease course.

As to rectal suction biopsy results: we know the results for certainly CF and certainly no CF. The values for patients with mild forms of CF are only recently being examined. How much residual activity of the CFTR protein would be expected with which mutation is thus not well known. And because certain of these mutations are so rare there is not a lot of information as to how the disease course would be. We also know that subjects with mild CFTR mutations can have a normal rectal suction biopsy result or a value very close to normal. Unclear results of the rectal biopsy could mean that either there is low but still normal CFTR activity or there is a very mild form of CF. This is dependent on the shape and the magnitude of the read out. Results of rectal biopsy in CF carriers are not well known. Abnormal suction biopsy results have been reported in people with only one CFTR mutation found. Because in routine testing only about 30 to 50 mutations are being examined one cannot be 100% certain that there is no second mutation present. With more sensitive test (sequencing the CFTR gene) in some of these persons a second mutation is indeed found. In the future we hope to learn more about the residual function of CFTR in rectal suction biopsy and nasal potential difference measurements in people with very mild symptoms.

Furthermore you ask whether the results from a rectal biopsy can change over time. There is more experience with repeated nasal PD results than with rectal suction biopsy. For nasal PD there is some variability of the test result but ‘the order of magnitude of the result’ and the conclusion of the result (normal or abnormal) stays the same.
I. Bronsveld, M. Proesmans, J. Dankert-Roelse, K. De Boeck

24.02.2009