CFTR and cystic fibrosis: We hypothesised that IRT levels may also reflect the severity of the CFTR gene defect among children with CRMS/CFSPID and would be different between children with CRMS/CFSPID who subsequently met the criteria for CF (CRMS/CFSPID→CF) and those whose diagnosis remained uncertain (CRMS/CFSPID→ CRMS/CFSPID).