INS and type 1 diabetes mellitus: Our data is therefore more in line with the prevalence of detectable proinsulin in the study of Steenkamp et al. (23) (16% in patient with undetectable postmeal C-peptide) compared with the T1D exchange residual C-peptide study group (89.9% in subjects without detectable C-peptide, and proinsulin not reacting to beta cell stimuli; 24).