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Methotrexate, plasma or serum

Lab area
Clinical Biochemistry - TDM & Toxicology
Method and equipment
Abbott Architect CI 4100, LC-MS/MS (Alternate assay)
Expected turn-around time
STAT/ Urgent/ Routine: 3 hours
Specimen type

Serum, Plasma (Heparin or EDTA)

Specimen requirements

250 uL

Storage and transportation

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

Shipping information
The Hospital for Sick Children
Rapid Response Laboratory
555 University Avenue, Room 3642
Toronto, ON
M5G 1X8
Phone: 416-813-7200
Toll Free: 1-855-381-3212
Hours: 7 days/week, 24 hours/day
Background and 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.

Disease condition

Anti-neoplastic, Anti-Rheumatic

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