How typical are false positives on cystic fibrosis tests?

Not Typical: Certain rare conditions can also lead to high sweat chloride levels (the gold standard test for cystic fibrosis). However, these rare conditions (there's a list) will cause the patient to exhibit different symptoms than those consistent with cystic fibrosis, so the sweat chloride test is considered to be reliable in diagnosing CF in conjunction with the patient's history and physical exam.

Which CF test: Depends on the type of CF test you are referring to. If it is newborn screening test (irt), then there are several conditions that can cause an elevated irt besides cf. For sweat chloride testing done by Pilocarpine iontophoresis, there are several rare conditions that can cause a false positive test. Please consult your local CF center or pediatric pulmonologist for more information.