While the lack of correlation among individuals could be due to the relatively small number of subjects in each group, recent studies in CF have indicated that altered sweat chloride and response to therapeutics are heterogeneous among individuals [21,38]; poor correlation between CFTR biomarkers (i.e. sweat chloride versus NPD) in the same individuals has also been problematic in these studies, which may explain why a tighter relationship is not apparent in this sample. Here, CFTR is linked to cystic fibrosis.