End of an era – decommissioning the Abbott TDxFlx following the assessment of two chemiluminescent methotrexate immunoassays.

Introduction: Fluorescence polarization assays (FPIA) for methotrexate (MTx) have been used since the 1980s to monitor high dose MTx toxicity and assess the need for “rescue” leucovorin therapy in neoplastic patients. Recently, two MTxchemiluminescent immunoassays (Abbott Diagnostics and ARK Diagnostics)have been developed. We assessed the assay performance of each method and the clinical comparability of patient MTx results, to enable decommissioning of the Abbott TDxFLx analyser in our laboratory.Methods: Methotrexate levels (0.05 – 60 mmol/L) were measured in neat and diluted(1:11 and 1:201) patient serum samples. Initially, patient (n=19) results from the Abbott (Architect i1000SR) and ARK (Roche Cobas 6000) assays were compared to results from the Abbott FPIA TDxFLx. A further patient sample evaluation (n=47), including intra-and inter-run variation and linearity assessment, was performed using the Abbott assay.Results: Over the analytical range tested, the Architect i1000SR MTx results had a higher probability (p = 0.18) of association with TDxFLx values, when compared to those from the Roche Cobas 6000 (p = 0.13). This was particularly evident in patient samples assayed neat or 1:11 diluted, with values < 1.50 μmol/L(Abbott: p = 0.39, ARK: p = 0.11). Patient MTx levels on the Architecti1000SR were highly correlated (r2=0.998), without significant bias (y=1.0542x- 0.0496) from TDxFLx values. At an MTx concentration of 0.44 μmol/L, the intra- and inter-run CVs for the Architect i1000SR method were 1.32% and1.54%, respectively.Conclusion: Abbott and ARK Diagnostic assays demonstrated satisfactory performance for monitoring toxicity in high-dose MTx regimens in our initial evaluation. The Abbott Architect MTx results, including those < 0.40 μmol/L, were linearly related to TDxFLx MTx levels. This assay also demonstrated low intra- and inter-run CVs and was implemented at the Royal Children’s Hospital to enable decommissioning of the Abbott TDxFLx analyser.