An evaluation of the Abbott IMx sirolimus assay in relation to a high-performance liquid chromatography-ultraviolet method

Abstract

Background: Sirolimus is an immunosuppressive agent used after organ transplantation. Monitoring its concentration is clinically relevant, but current methods of measurement using high-performance liquid chromatography (HPLC) are demanding. An immunoassay that uses readily available equipment offers a more convenient alternative. Methods: Blood specimens from transplant patients were assayed for sirolimus by an existing HPLC method with ultraviolet (UV) detection and by the Abbott microparticle enzyme immunoassay (MEIA). Within-run and between-day precision was assessed with duplicate analyses. Accuracy was assessed by comparison of results from the MEIA and HPLC assays, and from recovery experiments. Specificity was tested in specimens from patients taking alternative immunosuppressants. Results: Using specimens with sirolimus concentrations between 3.3 and 39.3 µg/L, within-batch and between-batch coefficients of variation (CVs) were 6.8% and 6.3%, respectively. Using control specimens with mean concentrations of 4.1, 12.1 and 25.4 µg/L, the respective CVs were 8.6%, 3.1% and 3.8 %. Using a Passing-Bablok regression technique, the regression equation was MEIA = 14; 1.26 x HPLC-1.97; the mean bias was 0.96 µg/L. Specimens from patients on cyclosporin and tacrolimus as the sole immunosuppressant had apparent sirolimus concentrations of <1 µg/L. Conclusion: The MEIA assay for sirolimus provides clinically useful information within an accessible format.