External quality assessment of Syva Emit and Abbott TDx II assays for methotrexate in serum

The Syva Emit and Abbott TDx II kits for determination of methotrexate in serum were compared using data from 14 samples distributed by the United Kingdom National External Quality Assessment Scheme to a mean of 38 European laboratories. For methotrexate concentrations above 0.2 mumol/L, there was no significant difference in the bias or coefficient of variation of measurements between the two techniques. A significantly greater number of measurements by Syva Emit (6.9%) were rejected as outliers > 3 SD from the sample mean compared with Abbott TDx (1.3%). The lower sensitivity of the Syva Emit assay was evident in data reported for samples containing methotrexate concentrations below 0.2 mumol/L.