Methotrexate, plasma or serum

Lab area

Clinical Biochemistry - TDM & Toxicology

Method and equipment

Equipment : Roche Cobas Pro c503

Method : The ONLINE TDM MTX assay is a homogeneous enzyme‑immunoassay. It is a two‑reagent system used for the detection of methotrexate in serum
and plasma. In this technology drug hapten attached to the enzyme glucose 6 phosphate dehydrogenase (G6PDH) serves as the binding partner to anti‑methotrexate antibody. A competitive reaction to a limited amount of specific anti‑methotrexate antibody takes place between the enzyme bound hapten and free methotrexate in the sample. Enzyme activity is reduced with bound antibody. Only active enzymes reduce NAD+ to NADH. The rate of NADH formation during the reaction correlates to the methotrexate concentration and is measured photometrically.

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 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

Canada

M5G 1X8

Phone: 416-813-7200

Toll Free: 1-855-381-3212

Hours: 7 days/week, 24 hours/day

Requisition

Methotrexate, plasma or serum - requisition

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