Abstract
OBJECTIVES: C-peptide is an equimolar by-product of insulin biosynthesis. It is used clinically to assess insulin secretion and differentiate types of diabetes. However, the lack of standardization across assays limits its broader application. This study aimed to examine discrepancies between the leading C-peptide measurement methods used in clinical laboratories and propose a solution to reduce them based on a complete traceability chain. METHODS: Two sets of serum samples were distributed to 10 manufacturers of C-peptide assays. The first set (A, n=20) was analyzed independently by each manufacturer, who then returned their results to us. Subsequently, we sent out the second set (B, n=20) along with the reference values for set A. For set B, each manufacturer provided both non-calibrated and recalibrated values for each sample. The recalibration was performed according to each manufacturer's internal standard protocols. We assessed how recalibration affected agreement between methods and alignment with the reference method. Non-parametric statistical approaches, including Passing-Bablok regression, level of agreement, and standard deviation analysis, were applied to compare data from multiple perspectives. RESULTS: Despite most manufacturers using the same WHO C-peptide calibrator material, significant disagreement was observed between methods prior to recalibration. Recalibration with matrix-appropriate serum samples reduced the discordance among assays, bringing them closer to the reference method. Overall, recalibration reduced both systematic bias and individual assay disagreement. CONCLUSIONS: These findings underscore the importance of appropriate calibration schemes to improve agreement across C-peptide assays, enhancing the accuracy of C-peptide testing for clinical practice.