MRM-based LC-MS method for accurate C-peptide quantitation.

Introduction: C-peptide secretion mirrors beta-cell function and has emerged as a valuable clinical biomarker for diabetes mellitus. C-peptide measurements can provide estimates of insulin secretory capacity, aiding in clinical decision-making and differentiation between diabetes types. Unfortunately, C-peptide assays are still not standardized, which may limit their practical clinical use. We have developed an MRM-based LC-MS method that demonstrated accuracy close to our reference method. Objective: To develop and validate a mass spectrometry method for accurate quantitation of C-peptide. Method: A serum sample was spiked with isotope-labeled C-peptide as a standard. The enrichment process involved protein precipitation with methanol, solid-phase extraction, and anion exchange for C-peptide enrichment followed by Glu-C digestion. The peptide LGGGPGAGSLQPLALE was quantitated using MRM in positive ion mode. The calibration process includes C-peptide CRM material to ensure a complete traceability chain for the measurement. Results: The assay exhibited linearity across a wide range of C-peptide concentrations and a limit of quantitation of 0.058 nmol/L. The inter-day imprecision was less than 9.6 % CV, and the intra-day imprecision was less than 8.9 % CV. Spiking with bilirubin, triglycerides, and hemoglobin demonstrated no interference, except for triglycerides at very high levels. The method exhibited a strong correlation to the C-peptide reference method (r2 = 0.95). Conclusion: The developed mass spectrometry method has demonstrated accurate results in C-peptide quantitation and can serve as a supplemental method to the existing C-peptide reference method. This ensures sustained stability over time and ultimately refines the existing reference system.