Enzymatic Assay for Methotrexate in Erythrocytes

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Abstract
SchrØder H, Heinsvig E-M. Enzymatic assay for methotrexate in erythrocytes. Scand J Clin Lab Invest 1985; 45: 657–659. Methotrexate (MTX) accumulates in erythrocytes in MTX-treated patients. We present a modified enzymatic assay measuring MTX concentrations between 10 and 60 nmol/1 in erythrocytes, adapted for a centrifugal analyser (Cobas Bio). About 40 patient's samples could be analysed within 1 h. The detection limit was 3 nmol/1. Within run and between-run precision was 7.4% and 13.5% for control 10 nmol/1 and 1.2% and 3.2% for control 50 nmol/1. Recovery was 85–115% of MTX added to haemolysed erythrocytes. We found the method useful for pharmacokinetic studies of MTX in erythrocytes in MTX-treated patients.