A chest x-ray may show bronchial wall thickening with either disease but the estimation of such thickening is semi-quantitative at best. CT scanning of the lungs is much more sensitive. One should not reach any conclusions about the diagnosis of asthma or cystic fibrosis (CF), on the basis of equivocal chest x-ray findings. Genetic testing is reliable but there is variability in the number of genes tested for from one laboratory to another, and thus it is conceivable that she may have an undetected CF gene, from her father.
She may have CF or asthma, both or neither and it behooves you and her doctor to have this clarified by a pediatric pulmonologist, preferably one with a special interest in CF.
You would be able to get a much more definitive answer to your question from any well-established, university medical center CF clinic doctor. I strongly suggest that you ask your granddaughter’s pediatrician for referral to a CF clinic. Everyone in your family will be less anxious, if you can get expert advice.
Good luck.
Have the sweat test repeated. I would definitely pay close attention to diet. Food Matters! I've been a respiratory therapist for 23 years and worked in pediatric intensive care for half of that time. If I can be of any support, please just let me know. This is a very difficult and scary time for your family, I'm sure. Many blessings to you and yours.