The standardization of C-peptide assays is highly desirable objective. Our groups have employed the use of a Certified Reference Material (CRM) to calibrate their respective measurement procedures and comparison studies have demonstrated good agreement between the two laboratories [Figure]. Although it was the original choice, The National Metrology Institute of Japan (NMIJ) CRM has not always been available. However, a new material from Cerilliant (C-161-0.1ML) proved equally well defined. Both materials include known amounts of pure C-peptide and small amounts of deamidated and pyroglutamated forms (Cerilliant Lot# FN0422195.6% modified; NMIJ CRM 6901-b, Lot# 058 1.9% modified) . The C-peptide CRMs from Cerilliant and from NMIJ behave identically when the mass concentration of the unmodified C-peptide is used to assign calibrator concentrations. To avoid potential bias, we conclude that the mass concentration of unmodified C-peptide (not the mass concentration of total C-peptide) in CRM should be used when making calibrators for mass spectrometry C-peptide assays to achieve comparable patient results. Variation in the relative amount of modified C-peptide is an obstacle to standardization, especially when comparing mass spectrometry-based assays with immunoassays, which use total C-peptide for calibration.
Z.Wu: Employee; Quest Diagnostics. K.Kabytaev: None. J.Mu: None. S.Connolly: None. M.J.Mcphaul: Employee; Quest Diagnostics. R.R.Little: None.