Is this an accredited CF clinic doing the sweat test? If not, I would ask to be referred to one. I was told by our first doctors (which were at a world renowned hospital system) that they can do them at their facility but they always just send cases to the other major hospital system because of the CF Clinic there. They do more and have more experience. Inadequate samples is a huge issue for infants as it is difficult to make them sweat. I believe a few moms on here had children who never had a positive sweat test and still were diagnosed via DNA testing. It can be tricky. It took over 4 times to get one for our son and it was in the hospital when he was very ill from malnutrition (he is pancreatic insufficient).
How old is you child again? Has your child been allergy tested? There is a test called a RAST test which tests for major allergens such as milk, wheat ect. You could place her/him (or yourself if breastfeeding) on an allergry free diet for a while and see what happens to her lung symptoms. Removing all dairy, wheat, eggs, nuts, soy, and corn products and then slowly adding them back after a period of 2-3 weeks, one by one can help to determine if she/he has an allergy and to which food. Not familiar with formula much, but there are some allergy free - dairy/soy free ones available (though costly). It might be worth it.