title: Feasibility of standardization of serum C-peptide immunoassays with isotope-dilution liquid chromatography-tandem mass spectrometry.
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 mug/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 mug/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.
authors.affiliation: Laboratory for Analytical Chemistry, Faculty of Pharmaceutical Sciences, Ghent University, Gent, Belgium.
authors.forename: Diego Rodríguez
authors.initials: DR
authors.lastname: Cabaleiro
authors.forename: Dietmar
authors.initials: D
authors.lastname: Stöckl
authors.forename: Jean M
authors.initials: JM
authors.lastname: Kaufman
authors.forename: Tom
authors.initials: T
authors.lastname: Fiers
authors.forename: Linda M
authors.initials: LM
authors.lastname: Thienpont
mesh_terms: D000293:Adolescent; D000328:Adult; D002096:C-Peptide / Q000097:blood*; D002853:Chromatography, Liquid / Q000592:standards; D005240:Feasibility Studies; D005260:Female; D006801:Humans; D007118:Immunoassay / Q000592:standards; D008297:Male; D013058:Mass Spectrometry / Q000592:standards; D008875:Middle Aged; D012015:Reference Standards
chemical_list: D002096:C-Peptide