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Paediatric Laboratory Medicine


Clinical Significance

Methotrexate (formerly Amethopterin) is an antimetabolite used in the treatment of certain neoplastic diseases, severe psoriasis, and adult rheumatoid arthritis. Chemically methotrexate is N-[4-[[(2,4-diamino-6-pteridinyl)methyl] methylamino]benzoyl]-L-glutamic acid.

Methotrexate inhibits dihydrofolic acid reductase. Dihydrofolates must be reduced to tetrahydrofolates by this >enzyme before they can be utilized as carriers of one-carbon groups in the synthesis of p.r.n. nucleotides and thymidylate. Therefore, methotrexate interferes with DNA synthesis, repair, and cellular replication. Actively proliferating tissues such as malignant cells, bone marrow, fetal cells, buccal and intestinal mucosa, and cells of the urinary bladder are in general more sensitive to this effect of methotrexate. When cellular proliferation in malignant tissues is greater than in most normal tissues, methotrexate may impair malignant growth without irreversible damage to normal tissues. 

The most frequently reported adverse reactions include ulcerative stomatitis, leukopenia, nausea, and abdominal distress. Other frequently reported adverse effects are malaise, undue fatigue, chills and fever, dizziness and decreased resistance to infection.

Test Name


Alternate Name/ Synonym

Trexall. Rheumatrex

Test Code


Drug Class



Therapeutic Drug Monitoring - TDM


Abbott TDx™, LC-MS/MS (Alternate assay)

External Proficiency Testing


Turn Around Time

Daily: 09:00h, 15:00h and 21:30h


Anti-neoplastic, Anti-Rheumatic

Specimen Type

Serum, Plasma (Heparin or EDTA)

Minimum Specimen Requirements

250 uL

Optimal Collection Time

Protocol Dependent


4°C (transport with a cool pack if possible)

Special Requirements

Please notify the laboratory (416-813-5906) if the patient has started on Carboxypeptidase rescue. Carboxypeptidase interferes with the FPIA immunoassay, so the analysis must be done by the alternative LC-MS/MS assay

Approval is not required

CPT Codes


Shipping and Contact Information

The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642
Toronto, ON
M5G 2G3
Phone: 416-813-7200
Phone: 1-855-381-3212

Therapeutic Range

Protocol Dependent