Hi Patricia!
Sweat tests are pretty accurate, especially when they are positive. Although I'm sure there is some degree of margin of error, pretty much everything I've read says that there are rarely false positives, but have slightly more false negatives.
I would guess they will want to run her genetics anyway, to add to her diagnosis and to understand at least a little better where she is at. The genes she has (there are over 1500 mutation of CF and to have CF you must have two) can't really predict how she will progress through the disease, but it is still useful to know. Knowing this information will help you to alleviate your concern of a false positive sweat result.
Many babies with CF only have digestive issues for some time. If she is on enzymes and zantac and still throwing up all her bottles, she might need to switch formula (I've heard they have formula studies that a CF baby can go on and get free formula) or maybe switch from zantac to prevacid. My daughter does great on the prevacid.
I'm glad to hear they have started her on percussions nice and young. Everything I've learned about CF has said that prevention is the key. Keep up the percussions now, because as she gets a little older she will fight them if she isn't used to it. My daughter was diagnosed at 5 months and by the time we got going on the percussions, she was screaming through them. Percussion sessions didn't get better until we got the vest when she was 15 months old.
Hang in there in the emotional arena. Diagnosis is tough on everyone. But coming here and learning all you can will make such a difference in the long run.
Take care!!
Heather