I know I'm a few months late on this and apologies for that.
The sweat test isn't totally accurate but gives some indication of whether cf might be present. People with cf sweat out 100% more salt than "normal" people so this test measures the amount of salt in the sweat. False negatives can occur if the test hasn't been carried out properly or the patient hasn't produced enough sweat to get an accurate reading. A genetic test would still need to be done after this to determine which genetic mutations of cf your daughter has. A lot of new medication coming out is targeted as specific CF mutations.
There is a genetic test available, two infact. The first one checks the genes for the most common CF gene mutations. It only tests for about 20 different mutations. The second test is more expensive but does a full genetic sweep and can check for a lot more mutations.
It sounds like your daughter has been struggling lots and could really do with some form of diagnosis even if it isn't CF so they know what they're treating.
I hope they find something that helps
All the best
Hey, I'm Gem with CF, age 28 and diagnosed at 2 years old. I'm from the UK so tell me if I say something you don't understand. I'm no expert but having CF myself I've learnt a few things along the way.
Diagnosed with: Cystic Fibrosis, Asthma, ABPA, Clinical Depression, Mild Liver Cirrohsis, Mild Osteopenia, Erythema Nodosum, Myopathy.