Received on November 28, 2007
Accepted on March 5, 2008

Endocrinology and Metabolism

Standardization of C-Peptide Measurements

¹ Department of Pathology & Anatomical Sciences, University of Missouri School of Medicine, Columbia, MO
² Department of Statistics, University of Missouri School of Medicine, Columbia, MO
³ Department of Medicine, Washington University School of Medicine, St. Louis, MO
⁴ Centers for Disease Control and Prevention, Division of Environmental Health Laboratory Sciences, Centers for Environmental Health, Chamblee, GA
⁵ Benaroya Research Institute, Seattle, WA
⁶ University of Washington and VA Medical Center, Seattle, WA
⁷ Department of Medicine and GCRC Analytical Core Laboratory, Albert Einstein College of Medicine of Yeshiva University, Bronx, NY

^* To whom correspondence should be addressed. E-mail: LittleR{at}health.missouri.edu.

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.