Physiology All India exam e8d900bf The best indicator of cystic fibrosis is A A positive family history of cystic fibrosis B The presence of digital clubbing C A sweat test with a chloride concentration of 70mEq/L D Bronchiectasis on a chest x-ray High-Yield Explanation A positive sweat test (i.e., a sweat chloride level greater than 60 mEq/L) is diagnostic of cystic fibrosis.