Feasibility of Standardization of Serum C-Peptide Im- munoassays with Isotope-Dilution Liquid Chromatog- raphy–Tandem Mass Spectrometry, Diego Rodrı́guez-

Background: Serum C-peptide concentrations reflect pancreatic function in different clinical and diagnostic settings; however, the utility of C-peptide testing is limited by the lack of standardized commercial immu-noassays. Standardization can best be done by split-sample comparison with a hierarchically higher refer-ence measurement procedure with a set of native sera. For serum peptides, isotope-dilution liquid chromatog-raphy–mass spectrometry (ID-LC/MS) is recommended as a reference measurement procedure. Methods: We evaluated the analytical performance characteristics of an ID-LC/tandem MS procedure for measurement of serum C-peptide after a 2-step solid-phase extraction. To investigate the feasibility of this procedure for use in standardization, we also performe