How common are false positives with cystic fibrosis tests?

Uncommon: False positive sweat tests to diagnose CF are very uncommon. Genetic testing should never have false positive results. However, newborn screening for CF is frequently positive in babies who have one copy of the CF gene (cf carriers). A positive newborn screening for CF is not a diagnosis of CF unless the baby has a positive sweat test or positive genetic testing with two mutations identified.

Rare: Genetic testing has a very low false positive rate, sweat cloride i less precise.

DNA vs. sweat tests: For sweat tests, do not trust a laboratory which is not accredited by the Cystic Fibrosis Foundation. "Sweat tests should be done at a Cystic Fibrosis Foundation-accredited care center". See http://www.cff.org/AboutCF/Testing/