Once a test result is positive, it is always positive.
Nothing changes as a patient grows older.
And sweat test values do not vary when the patients have colds or other temporary illnesses. I need to stress the importance of the laboratory doing the tests. Most CF centers find that CF has been misdiagnosed in about half of all of the patients they see who have been tested by inexperienced laboratory personel. Sweat test laboratories associated with an approved CF center, have passed accreditation by the national CF Foundation, and can be trusted. Many other laboratories are also good, but it is harder to know about those not in CF Centers.