1418 Correspondence CHEST Correspondence Understanding the Relationship

In a recent issue of CHEST (January 2013), Durmowicz et al 1 highlight the fi nding that despite improvement in both cystic fi bro-sis (CF) transmembrane conductance regulator (CFTR) and pul-monary function in clinical trials of ivacaftor (recently approved by the US Food and Drug Administration for individuals with CF and the G551D mutation), there does not appear to be a direct correlation between changes in sweat chloride (a measure of CFTR function) and changes in FEV 1 (an established effi cacy measure). They also were unable to identify a minimum threshold for change in sweat chloride that was associated with FEV 1 improvement. Despite the fi ndings, 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 chlorid