Methotrexate

Norm    
24 hours from the start of infusion less than 10µmol/l
48 hours from the start of infusion less than 0.5–1.0µmol/l
72 hours from the start of infusion less than 0.05–0.10µmol/l
If elimination is impaired, continue testing until the specified level is reached, less than 0.05–0.10µmol/l

Synonyms: Methotrexate, Trexan^®.
Serum
Red cap

Methotrexate is an antineoplastic cytostatic. It disrupts DNA synthesis in the body. When treating oncological diseases with methotrexate, TDM is an important part of the treatment process. At the beginning of treatment, for a short time, methotrexate serum concentration exceeds the toxicity threshold by several thousand times. It is important to monitor treatment so that the methotrexate concentration is below a certain threshold at specific time intervals: 24 hours, 48 hours, 72 hours. The most important factor determining methotrexate toxicity is the drug concentration in the serum at a specific time. If necessary, if the decrease in drug concentration was not achieved at the right time, an antidote can be administered. The toxicity of the drug can be reduced by administering 5-formyltetrahydrofolate, calcium folinate. This substance is also known as Citrovorum factor.

Approximately 40–50% of the drug (if a low dose of methotrexate is administered) is excreted unchanged in the urine within the first 48 hours after injection. If a higher dose is administered, then up to 90% of the drug is excreted in the urine within the first 48 hours. The majority of the drug remaining in the blood is eliminated within the first 8 hours after administration. The main toxicity is bone marrow suppression with megaloblastic erythropoiesis. When taken with salicylates, toxicity may increase due to the unwanted effect of methotrexate on the kidneys. If drug elimination is impaired, the decrease in blood concentration may be delayed.

Source | Handbook of Basic Laboratory Tests | Doctor of Medical Sciences Gintaras Zaleskis