Hello. My son has had two DNA genetic tests done. The first with Genzyme (32 panel) with no found mutations. Than he went on to have the amplified Ambry test and still came back with no known mutations. He has had two positive sweat tests. His pulmo diagnosed him with CF and is not concerned right now with finding my son's mutations.
I think the doctor looks at the "whole picture" when diagnosing their patients.