How low is really low? Comparison of two C‐peptide assays to establish residual C‐peptide production in type 1 diabetes

INTRODUCTION: C‐peptide is an important marker to assess residual insulin production in individuals with type 1 diabetes (T1D). The accuracy and detection limits of C‐peptide assays are important to detect C‐peptide microsecretion and to reliably observe changes over time in these people. We compared and verified two commercially available assays able to measure C‐peptide in the picomolar range. METHODS: The ultrasensitive Mercodia enzyme‐linked immunosorbent C‐peptide assay (ELISA) was compared with the Beckman immunoradiometric assay (IRMA) for C‐peptide, assessing reproducibility (coefficient of variation [CV]), limit of blank (LoB), limit of detection (LoD) and limit of quantitation (LoQ). RESULTS: For both assays within‐run and between‐run variation were high at the low (around the detection limit) C‐peptide concentration range, with CVs of around 40%. LoB values for the ultrasensitive ELISA and the IRMA were 1.3 and 0.16 pmol/L respectively. LoD values were 2.4 and 0.54 pmol/L respectively. LoQ values were 9.7 and 3.8 pmol/L respectively. Only the IRMA met the specifications claimed by the manufacturer. CONCLUSIONS: The IRMA provided the lowest threshold for quantification of serum C‐peptide. LoQ of commercially available assays should be established in‐house before applying them in research studies and clinical trials in which low C‐peptide levels have clinical or scientific relevance.