Determination of sweat electrolytes has been performed in clinical laboratories for over 40 years and remains the gold standard diagnostic test for CF.143 The measure provides a sensitive indicator of CFTR activity, and correlates well with the CF phenotype.120 Sweat chloride measurements are feasible in multicenter clinical trials,5,121,122,143 and use of the macroduct collection system allows analysis in a central laboratory, facilitating standardized methodology among centers. This evidence concerns the gene CFTR and cystic fibrosis.