Despite the findings, lack of a linear correlation between the changes in these outcome measures should not diminish the potential utility of sweat chloride for predicting clinical outcomes. Although the data presented failed to show a population level threshold for change in sweat chloride, the absolute sweat chloride concentrations after treatment in the two ivacaftor trials were not assessed. While adult sweat chloride values are highly variable across CF and healthy control subjects,2 concentrations < 60 mM are typical among individuals without CF, and intermediate values are associated with a more benign clinical course. In reanalysis of the ivacaftor trial data, Seliger et al3 show that nearly every participant has a resulting sweat chloride ≤ 80 mM after 2 weeks of treatment, which does not provide the spectrum of response required to readily establish a linear correlation with another outcome. Thus, correlation between changes in sweat chloride and change in FEV1 may not fully capture the predictive ability of sweat chloride.