title: Standardization of C-peptide measurements.
abstract: BACKGROUND
  C-peptide is a marker of insulin secretion in diabetic patients. We assessed within- and between-laboratory imprecision of C-peptide assays and determined whether serum calibrators with values assigned by mass spectrometry could be used to harmonize C-peptide results.
  
  
  METHODS
  We sent 40 different serum samples to 15 laboratories, which used 9 different routine C-peptide assay methods. We also sent matched plasma samples to another laboratory for C-peptide analysis with a reference mass spectrometry method. Each laboratory analyzed 8 of these samples in duplicate on each of 4 days to evaluate within- and between-day imprecision. The same 8 samples were also used to normalize the results for the remaining samples to the mass spectrometry reference method.
  
  
  RESULTS
  Within- and between-run CVs ranged from <2% to >10% and from <2% to >18%, respectively. Normalizing the results with serum samples significantly improved the comparability among laboratories and methods. After normalization, the differences among laboratories in mean response were no longer statistically significant (P = 0.24), with least-squares means of 0.93-1.02.
  
  
  CONCLUSIONS
  C-peptide results generated by different methods and laboratories do not always agree, especially at higher C-peptide concentrations. Within-laboratory imprecision also varied, with some methods giving much more consistent results than others. These data show that calibrating C-peptide measurement to a reference method can increase comparability between laboratories.
authors.affiliation: Diabetes Diagnostic Laboratory, Department of Pathology & Anatomical Sciences, University of Missouri School of Medicine, Columbia, MO 65212, USA. LittleR@health.missouri.edu
authors.forename: Randie R
authors.initials: RR
authors.lastname: Little
authors.forename: Curt L
authors.initials: CL
authors.lastname: Rohlfing
authors.forename: Alethea L
authors.initials: AL
authors.lastname: Tennill
authors.forename: Richard W
authors.initials: RW
authors.lastname: Madsen
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Polonsky
authors.forename: Gary L
authors.initials: GL
authors.lastname: Myers
authors.forename: Carla J
authors.initials: CJ
authors.lastname: Greenbaum
authors.forename: Jerry P
authors.initials: JP
authors.lastname: Palmer
authors.forename: Eduard
authors.initials: E
authors.lastname: Rogatsky
authors.forename: Daniel T
authors.initials: DT
authors.lastname: Stein
mesh_terms: D002096:C-Peptide / Q000097:blood*; D002138:Calibration; D002853:Chromatography, Liquid; D006801:Humans; D007118:Immunoassay / Q000379:methods; D016018:Least-Squares Analysis; D013058:Mass Spectrometry / Q000592:standards; D012015:Reference Standards
chemical_list: D002096:C-Peptide