Re. the CF test that was done initially that turned up negative:
I missed your mention that your child had a normal NBS for CF. As recently as 6yrs ago many states were not even screening for CF yet! My state was one of the last to mandate it and this only occurred in the last year. However, a newborn screen is just that- a screen, and is not diagnostic. Especially 6 years ago, it likely did not include any genetic testing but was rather more likely to only be a test of IRT levels. The majority of babies with elevated IRT levels do not have CF and some that do have it will not present with elevated IRT (and therefore don't get further testing if asymptomatic at that time) so some will be missed. We see this happen quite often here on this website; children who fall through the cracks of this system and often go quite a long time before diagnosis. It's very, very fortunate when a dr is observant and proactive enough to pursue testing~ CF is often NOT on their radar and contributes to dx coming very late and after damage is already done.
In my daughter's case she was not dx'ed until age 7, and this was very much to her detriment. She lost a lot of ground compared to where her health was as a toddler/preschooler. She was in growth failure by the time she was dx'ed and had started experiencing more frequent respiratory infections that would have benefited so much by appropriate, aggressive management.