Monitor therapeutic drug level, evaluate potential toxicity
No precise relationship between methotrexate serum levels and antineoplastic efficacy has been established. Following a 4-6 hour intravenous methotrexate infusion with dosages ranging from 50 mg/m2 - 15 g/m2, a patient with a 24 hr serum level > 5 - 10 umol/L, a 48 hr serum level > 0.5 - 1.0 umol/L, and a 72 hr serum level > 0.2 umol/L is at an increased risk of toxicity if conventional low dose leucovorin rescue is given. In this case, high dose leucovorin rescue would be indicated.
Collect blood in a red top or green top tube; do not use SST tube. Centrifuge, aliquot serum into a plastic vial, and refrigerate. Cerebrospinal fluid is also an acceptable specimen.