As Melissa notes, the sweat test is not always determinant, especially with some variants of the disease. Both of my girls have CF; one tested borderline (49) and the other tested normal (11). On genetic testing (the full Ambry panel), both had an atypical form of CF. However, their symptoms were more respiratory than digestive; I believe that is the case with many of the atypical cases, but I don't know for sure. There is a lot of variety in the genetics of the disease.
No one wants to get this diagnosis, but in our experience, it was a very good thing that we kept testing. Getting them on therapies has really helped them stay healthy.