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Methotrexate (MTX) is used to treat a variety of malignancies including ALL, lymphoma, choriocarcinoma, osteogenic sarcoma and carcinomas of the breast, lung and head and neck. It may be administered orally or intravenously. High dose therapy may be associated with significant toxicity. Therapeutic monitoring is essential to guide the timing and amount of leucovorin (folinic acid) rescue.

The serum concentration of MTX is usually monitored at 24, 48 and, if necessary, 72 hours after a single dose. The toxic concentrations at each time point are as follows:

Time after Single Dose

Toxic concentration

24 hours

>4.9 umol/L

48 hours

>0.49 umol/L

72 hours

>0.049 umol/L

MTX is eliminated primarily by the kidneys. ; 70 – 90% is excreted unchanged. Clearance is reduced in patients with renal disease, ascites or pleural effusions

One red top tube of blood should be drawn at the times specified in the table. Spinal fluid can also be tested.

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