Feasibility of standardization of serum C-peptide immunoassays with isotope-dilution liquid chromatography-tandem mass spectrometry

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 immunoassays. Standardization can best be done by splitsample comparison with a hierarchically higher reference measurement procedure with a set of native sera. For serum peptides, isotope-dilution liquid chromatography-mass spectrometry (ID-LOMS) 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 performed a method comparison with 3 representative commercial assays. Results: The ID-LC/tandem MS procedure showed maximum within-run, between-run, and total CVs on dedicated sera (C-peptide concentrations, 1.6 and 4.0 mu g/L) of 2.1%, 2.5%, and 2.9%, respectively; an accuracy of 94.6% - 104.1%; a minimum trueness of 98.1% (95% confidence interval, 96.2% - 100.0%), and limits of quantification and detection of 0.15 and 0.03 mu g/L, respectively. Deming linear regression analysis of the method-comparison data showed that the immunoassays correlated well with ID-MS and were specific, but lacked intercomparability and trueness. We propose that the deficiencies can be resolved by recalibration on the basis of the method comparison. Conclusions: The ID-LC/tandem MS procedure is suitable for specific and accurate measurement of basal and stimulated serum concentrations of proinsulin C-peptide fragment 33-63 and is suitable for use in standardization of C-peptide immunoassays. (c) 2006 American Association for Clinical Chemistry.