Why Monitor Kidney Function on Methotrexate?
Methotrexate can harm kidneys, especially at high doses or in patients with pre-existing issues. Regular tests detect early damage, as the drug is excreted through the kidneys and can cause acute toxicity if function declines.[1][2]
How Often Are Kidney Tests Recommended?
Guidelines call for baseline kidney tests (serum creatinine, BUN, GFR) before starting methotrexate, then every 1-3 months during treatment, or more frequently if doses exceed 20 mg/week, in elderly patients, or with risk factors like dehydration or NSAID use. Adjust frequency based on results—weekly if creatinine rises.[1][3]
What Tests Are Used?
- Serum creatinine and estimated GFR (most common).
- Blood urea nitrogen (BUN).
- Urine analysis for protein or casts.
Folinic acid rescue and hydration reduce risks in high-dose regimens.[2][4]
Who Needs Tests Most?
Higher risk groups include:
- Elderly or dehydrated patients.
- Those with baseline GFR <60 mL/min.
- High-dose users (e.g., cancer therapy vs. low-dose for rheumatoid arthritis).
Low-dose weekly therapy (e.g., 7.5-25 mg for RA) still requires monitoring, but toxicity is rarer.[1][3]
What Happens If Kidney Function Drops?
Dose reduction or discontinuation if creatinine rises >50% or GFR falls significantly. Symptoms include fatigue, swelling, or reduced urine output. Untreated, it risks permanent damage or life-threatening toxicity.[2][4]
How Does Methotrexate Affect Kidneys?
It inhibits dihydrofolate reductase, leading to crystal formation in tubules and reduced excretion. Alkalinization of urine (with sodium bicarbonate) prevents this in high-dose settings.[1][2]
Sources:
[1]: American College of Rheumatology Guidelines
[2]: UpToDate: Methotrexate Toxicity
[3]: NICE Guidelines: Methotrexate Monitoring
[4]: Lexicomp: Methotrexate Dosing