Cystic fibrosis is an autosomal recessive disease involving CFTR (which encodes the CFTR protein), which means you need a double hit to express the disease. If the genetic test only picked up one, then it must have missed the other.

The reason something is an “autosomal recessive” disease is because the protein encoded by the gene (of which you have two alleles, of course) does something where as long as you make SOME protein, your body should be okay.

That’s kind of vague, so take the case of Cystic Fibrosis: you don’t present with Cystic Fibrosis if you have at least one functional allele -- that’s because CFTR protein is a protein that (in the case of bronchiole tissue) moves Cl- from inside cells to the outside lumen, which brings with it H2O and keeps the bronchiole lumen nice and watery, and fluid and non-viscous and non-pluggy.

So long as you make enough of this protein, you don’t “need” both alleles to be good; the good allele can “make up for” (make enough of the protein product to compensate for) the “broken allele.” So, once again, understanding the pathophys of a disease allows you to reason through and predict things like disease penetrance and expressivity.

does not need to be the same mutation recall CF has many mutations that cause it so likely this patient has 2 different mutations both of which led to her having CF and the test was specific only for one of her mutations

To those wondering how a CFTR mutation is not detected:

CF can result from more than a thousand different mutations.[45] As of 2016, typically only the most common mutations are tested for, such as ΔF508[45] Most commercially available tests look for 32 or fewer different mutations.

Wikipedia

AR disease. She's got all the symptoms, she must have both defective genes. Genetic testing for the 36 most common mutations shows a detectable mutation (G551D) in one allele of the CFTR gene. What about the 2nd gene... we KNOW she has the disease. It wasn't detected.