Response to Comment on Sims et al. Proinsulin Secretion Is a Persistent Feature of Type 1 Diabetes. Diabetes Care 2019;42:258–264

In a cohort of 319 individuals with long-duration type 1 diabetes (T1D), we found that 89.9% of participants with undetectable stimulated C-peptide had measurable fasting or stimulated serum proinsulin (1). To ensure the validity of our results, we performed rigorous validation of the Millipore human intact and Des (31,32) proinsulin radioimmunoassay that included spike-in experiments using insulin, C-peptide, and proinsulin; independent calculation of a lower limit of detection; assessment of inter- and intra-assay coefficients of variation; analysis of linearity of dilution; analysis of samples pre- and postpancreatectomy; analysis of effects of increased insulin autoantibody titers; and quantitative validation of our findings using mass spectrometry (1). The letter by Steenkamp et al. (2) in response to our article suggests the Millipore assay may overestimate proinsulin levels compared with the ALPCO STELLUX human total proinsulin ELISA. The authors’ concerns are based on their finding that, using the ALPCO assay, proinsulin was detected in only 16% of random samples from a smaller subset of C-peptide–negative individuals from the same T1D Exchange Residual C-peptide Study (3). Quantitative immunoassay performance differences between different assay platforms are not surprising due to antibody interactions with different epitopes on native antigens …