Enzymatic assay for methotrexate in erythrocytes.

Methotrexate (MTX) accumulates in erythrocytes in MTX-treated patients. We present a modified enzymatic assay measuring MTX concentrations between 10 and 60 nmol/l 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/l. Within run and between-run precision was 7.4% and 13.5% for control 10 nmol/l and 1.2% and 3.2% for control 50 nmol/l. 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.