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Subject: Methotrexate

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  • Folic acid antagonist. Other names: Folex, Mexate, Trexall.

  • Normal range: TDM generally performed to ensure plasma/serum concentration <1 μmol/L at 48 hours postinfusion and <0.1 μmol/L at 72 hours postdose


  • Methotrexate is an antineoplastic drug used solely or in combination with other antineoplastic drugs for the treatment of leukemia and other diseases.

  • Severe psoriasis, sarcoidosis, and granulomatosis have been treated with methotrexate in relatively low doses.

  • High-dose methotrexate (greater than approximately 20 mg/kg body weight) with citrovorum factor rescue have been used with favorable results in the treatment of osteogenic sarcoma, leukemia, non-Hodgkin lymphoma, lung cancer, carcinoma of the head and neck, and breast cancer.

  • The efficacy of methotrexate in the treatment of other tumors such as prostate cancer is being investigated.


  • Potential toxicity: Reported therapeutic and toxic ranges are dependent on dose and the time the sample is drawn postdose. Consult protocol to assess toxicity.

    • 24 hours: >10 μmol/L

    • 48 hours: >1.0 μmol/L

    • 72 hours: >0.1 μmol/L


  • Immunoassay-based tests (EMIT, FPIA) for serum/plasma

  • Serum must be collected in tubes without serum separator gel.

  • Cells must be separated as soon as possible after collection.

  • Heparinized, EDTA, and fluoridated collection tubes for plasma are acceptable.

  • This assay measures the total (protein bound and free) levels of methotrexate in serum and plasma.

  • It is known that aminopterin and APA (a metabolite of methotrexate) cross-react significantly with the EMIT assay and less so with the FPIA.

  • Patients should be rebaselined when changing the methodology.

  • Samples are stable for up to 24 hours when refrigerated and protected from light.