Evaluation of a fluorescent polarization immunoassay for whole blood everolimus determination using samples from renal transplant recipients

Objectives: This study compared the performance of a fluorescent polarization immunoassay (FPIA) against HPLC-tandem mass spectrometry (HPLC-MS) for the measurement of everolimus in renal transplant recipients. Design and methods: A total of 333 pre-dose samples from 45 renal transplant patients were analyzed by FPIA and HPLC-MS. Results: The inter-batch inaccuracy and precision of the FPIA for control samples were ≤6% and ≤13.0%, respectively (n = 17). The comparison of patient results yielded the Deming regression equation FPIA = 1.19 × HPLC-MS + 0.51. The mean bias was 24.4% (95% CI: −3.0 to 54.2%, range: −30.1% to 79.4%). Conclusions: The FPIA had acceptable analytical performance during the study but compared to HPLC-MS overestimated everolimus in patient samples. This overestimation is probably due to calibration differences between the methods and cross-reactivity of the FPIA antibody with everolimus metabolites. The clinical importance of the observed overestimation by FPIA requires further investigation