Which laboratory values are critical to monitor for methotrexate toxicity?

• A. Renal function alone.
• B. Renal function (creatinine) and methotrexate serum trough level.
• C. Liver enzymes alone.
• D. Platelet count alone.

Answer: (B)

The essential idea is that methotrexate toxicity hinges on how much drug remains in the body, which depends on kidney function and the actual drug exposure. Methotrexate is cleared primarily by the kidneys, so assessing renal function (creatinine/creatinine clearance) shows how well the drug will be eliminated; impaired renal function raises the risk of accumulation and toxicity. At the same time, the methotrexate trough level tells you how much drug is still in circulation at a given time, guiding the intensity of rescue measures (like leucovorin) and adjustments to hydration or alkalinization to prevent toxicity.

Relying on renal function alone doesn’t capture the current exposure, since people with similar creatinine can have different MTX levels. Liver enzymes or platelet counts can reflect effects of MTX but do not predict clearance or immediate toxicity risk as effectively. So monitoring both renal function and the MTX trough level provides the most complete picture for safety.