Standardization of C-peptide measurements.
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.
Little
Randie R
RR
Diabetes Diagnostic Laboratory, Department of Pathology & Anatomical Sciences, University of Missouri School of Medicine, Columbia, MO 65212, USA. LittleR@health.missouri.edu
Rohlfing
Curt L
CL
Tennill
Alethea L
AL
Madsen
Richard W
RW
Polonsky
Kenneth S
KS
Myers
Gary L
GL
Greenbaum
Carla J
CJ
Palmer
Jerry P
JP
Rogatsky
Eduard
E
Stein
Daniel T
DT
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
D002096:C-Peptide