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Clinical Chemistry 52: 1193-1196, 2006. First published April 13, 2006; 10.1373/clinchem.2005.062505
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(Clinical Chemistry. 2006;52:1193-1196.)
© 2006 American Association for Clinical Chemistry, Inc.

Technical Briefs

Feasibility of Standardization of Serum C-Peptide Immunoassays with Isotope-Dilution Liquid Chromatography–Tandem Mass Spectrometry

Diego Rodríguez Cabaleiro1, Dietmar Stöckl1, Jean M. Kaufman2, Tom Fiers2 and Linda M. Thienpont1,a

1 Laboratory for Analytical Chemistry, Faculty of Pharmaceutical Sciences, Ghent University, Gent, Belgium;2 Laboratory for Hormonology and Department of Endocrinology, Ghent University Hospital, Gent, Belgium;

aaddress correspondence to this author at: Laboratory for Analytical Chemistry, Faculty of Pharmaceutical Sciences, Ghent University, Harelbekestraat 72, B-9000 Gent, Belgium; fax 32-9-264-81-98, e-mail linda.thienpont{at}ugent.be


Abstract

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 split-sample comparison with a hierarchically higher reference measurement procedure with a set of native sera. For serum peptides, isotope-dilution liquid chromatography–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 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 µg/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 µg/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.




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