Real‐world data confirm elexacftor/tezacaftor/ivacaftor modulators halves sweat chloride concentration in eligible people with cystic fibrosis

Sweat chloride concentration, a diagnostic feature in cystic fibrosis (CF), reflects CF transmembrane conductance regulator (CFTR) activity. CFTR modulator therapies, especially elexacaftor/tezacaftor/ivacaftor (ETI), has improved CF outcomes. We report nationwide, real‐world data on sweat chloride concentration in people with CF (pwCF) with and without modulator therapies. All Danish pwCF with a minimum of one F508del allele were included. Sweat chloride measurements were stratified by genotype and modulator treatment. Differences were assessed using mixed‐effects models. We included 977 sweat chloride measurements from 430 pwCF, 71% of which were F508del homozygous. Heterozygous and homozygous ETI‐treated pwCF had an estimated mean sweat chloride concentration of 43 mmol/L (95% confidence interval: 39; 48) and 43 mmol/L (39; 47), respectively—48% and 59% lower than those without treatment. High variation in concentrations remained regardless of treatment status. Despite ETI treatment, 27% heterozygous and 23% homozygous pwCF had elevated concentrations (≥60 mmol/L). These real‐world data confirm a substantial decrease in sweat chloride concentration during modulator treatment, especially ETI, where mean concentrations halved. However, large variation remained, including persistently high concentrations. These findings emphasize the potential of sweat chloride concentration as a treatment response biomarker and the need to explore its heterogeneity and relationship with clinical outcomes.