Ivacaftor, a cystic fibrosis transmembrane regulator (CFTR) potentiator, decreased sweat chloride concentrations and improved clinical measures in cystic fibrosis (CF) patients with the G551D mutation.

Sweat chloride measurements at day 15 had an overall positive predictive value (PPV) of 86.3%, a negative predictive value (NPV) of 65.5%, sensitivity of 73.9%, and specificity of 80.9% for an FEV₁ improvement of ≥ 5% from baseline at week 16. For ivacaftor patients the median FEV₁ improvement was 16.7%; for placebo patients 0.4%. For patients aged 6–11 years who received ivacaftor and who had a sweat chloride decrease of ≥ 40 mmol/L from baseline at day 15, a median weight gain of 11.2% at week 16, compared to 6% for those with a smaller decrease was observed.

Changes in sweat chloride concentration at day 15 following treatment with ivacaftor may have sufficient predictive potential to identify individuals that show improvement in pulmonary function and weight gain after 16 weeks of treatment.