CFTR and cystic fibrosis: Sweat chloride is a direct, sensitive measure of CFTR function, is a well established diagnostic measure of cystic fibrosis, and predicts disease severity at a population level in natural history studies, although it has not been used to prospectively predict individual clinical benefit.9,25,26 Therefore, although new CFTR modulator therapies should demonstrate at least non-inferiority in terms of FEV1 % predicted improvement compared with standard of care, improvement in sweat chloride concentration allows an opportunity to differentiate between CFTR modulator regimens.