Evaluation of a new immunoassay to measure sirolimus blood concentrations compared to a tandem mass-spectrometric chromatographic analysis.

A new immunoassay of sirolimus based on the microparticle enzyme immunoassay (MEIA) principle has been developed with collaboration of Abbott Diagnostics. Laboratories and Axis-Shield. Our laboratory evaluated this new assay on 153 whole blood samples (EDTA) drawn from a population of renal (n = 141) and hepatic (n = 12) transplant patients. Each blood sample was analyzed simultaneously by MEIA (Y) and by a reference method (X) used routinely in our laboratory, namely, liquid chromatography tandem mass spectrometry (LC-MS/MS). The statistical analysis of Passing-Bablok produced the following results: Spearman r value = 0.95, slope = 1.15 and intercept with the Y axis = +0.2 ng/mL. The observed global overestimation of 15% compared to the reference method could be explained by the cross-reactivity of sirolimus metabolites with the antibody. A complementary analysis taking into account the transplanted organ (kidney versus hepatic) did not show any significant difference between the populations, most likely owing to the low number of hepatic transplantation samples. The analytical performance of the MEIA method showed CV < or =10% and a limit of quantification of 1.5 ng/mL, which were acceptable for routine clinical monitoring. In conclusion, the MEIA method has shown robust, stable, reproducible, features with an excellent correlation with the reference LC-MS/MS method.