I dont like how the doctor worded things with a "mutated form of cf". The gene mutations that cause CF are defected which basically means "everyone per say" carries CF genes, but only the ones who carry the "defective" genes that actually cause the illness are considered carriers. Failure to thrive, difficulty gaining weight etc are common symptoms with many of the known gene mutations for CF. Personally I dont think HOW the doctor said it matters. What it comes down to is that your daughter & SIL are known carriers and your grandchild has symptoms indicating she might actually have CF. A sweat test is the usual first step tho since the parents know their gene mutations, I would think it would be safer/more reliable to have your granddaughter tested for these 2 gene mutations. Sweat tests are becoming less reliable or apparently so I should say!