Application of a C-peptide electrospray ionization-isotope dilution–liquid chromatography–tandem mass spectrometry measurement procedure for the evaluation of five C-peptide immunoassays for urine

This study applied electrospray ionization-isotope dilution–liquid chromatography–tandem mass spectrometry for the evaluation of five urinary C-peptide immunoassays via split-sample measurements. The immunoassays measured in duplicate in the same run, the comparison method in triplicate over different runs. From the data, the within-run imprecision and the method comparison total RSDs were calculated. Regression analysis revealed on the one hand systematic differences, on the other, an excellent correlation between the test and comparison methods. From the spread of the data around the regression line in comparison with the 95% prediction intervals from the total RSD, sample-related effects and/or specificity problems were apparent and investigated.