Methotrexate
Measures how much methotrexate is in your blood at a set time after a dose. It is mostly used to guide care after high-dose treatment.
AI-verified against cited sources. Awaiting expert reviewAlso called MTXLOINC 14836-1
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What it is
Methotrexate is a medicine used for some cancers and for conditions such as rheumatoid arthritis and psoriasis. This test measures how much of the drug is in your blood at the moment of the draw. The result only makes sense alongside the time of your last dose, because the level falls as your body clears the drug, mainly through the kidneys.
Why it matters
After high-dose methotrexate, care teams check levels at set times to see whether the drug is leaving the body on schedule. If the kidneys slow down, the drug can linger and affect the bone marrow, gut, liver and kidneys, so the team may adjust fluids or give rescue treatment. With the weekly low doses used for arthritis or psoriasis, methotrexate levels aren't usually part of routine care. Blood counts, liver tests and kidney tests are the usual checks.
If it’s low
A low or not-measurable level is expected when you aren't receiving methotrexate, or when many hours or days have passed since a dose. During high-dose treatment, a level that falls on schedule is the expected pattern. Some tests also can't measure the small amounts seen with low-dose treatment.
Common causes
- Many hours or days since the last dose
- Not currently taking or receiving methotrexate
- Low-dose treatment, where levels can be below what some tests measure
- A sample drawn at a different time than the plan intended
What can help
- Ask your care team whether a methotrexate level is needed for your treatment plan at all
If it’s high
A high result means more methotrexate is in your blood than the plan expects at that hour. What counts as high depends on the treatment plan and the hours since the dose, so the same number can be expected at one time and above target at another. Your care team reads each result against a time-based chart.
Common causes
- Slower clearance through the kidneys, including dehydration or kidney injury
- Fluid collections such as ascites or pleural effusion, which release the drug slowly
- Medicines that raise methotrexate levels, such as some penicillin or sulfonamide antibiotics, proton pump inhibitors, probenecid, and aspirin or other NSAIDs
- A sample drawn earlier after the dose than the plan assumed
- A test method that reads falsely high after the rescue medicine glucarpidase
What can help
- Ask your care team how this result compares with the target for your treatment plan
- Make sure your care team knows every medicine, supplement and over-the-counter pain reliever you take
- Ask whether your kidney function and fluid intake are being checked alongside this level
What can affect your result
- Time since the dose: levels fall over hours to days
- Kidney function and hydration, since methotrexate leaves the body mainly through the kidneys
- Fluid collections in the body (ascites, pleural effusion) that hold on to the drug
- Other medicines, including NSAIDs, proton pump inhibitors, penicillins and probenecid
- Test method: immunoassays and chromatographic methods can differ, especially at low levels
- After the rescue medicine glucarpidase, a by-product (DAMPA) makes immunoassays read falsely high for up to about 48 hours; a chromatographic method (HPLC or LC-MS/MS) is preferred in that window.
- Immunoassays cross-react with DAMPA and have a lower limit of about 20 nmol/L, so they may not measure the small amounts seen with low-dose treatment.
- The hours since the dose change what a result means; a sample drawn at a different time than the protocol expects can't be compared with its cut-offs.
- Fluid collections such as ascites or pleural effusion hold on to methotrexate and release it slowly, which can keep levels up for longer.
Set by the treatment protocol. After a high-dose infusion, samples are drawn at fixed hours after the infusion starts or ends (for example 24, 48 and 72 hours, depending on the protocol). Levels fall over hours to days after a dose. The terminal half-life is about 3 to 10 hours with low-dose treatment and 8 to 15 hours after high-dose IV treatment, and it is longer when kidney function is reduced.
When to talk to a clinician
If you take methotrexate, contact your care team promptly for fever, new mouth sores, severe vomiting or diarrhea, easy bruising or bleeding, or passing very little urine. If you are on a high-dose protocol, your treating team follows these levels as they come back and will tell you the targets. If you feel very unwell (for example, chest pain, trouble breathing, confusion or fainting), call 911 or your local emergency number.
Questions you could ask
- Which blood tests do you use to monitor my methotrexate treatment?
- How often should those tests be repeated?
- What level were you aiming for at this time after my dose?
- Should my kidney function and fluids be checked more closely?
- Could any of my other medicines be raising my methotrexate level?
- Is a methotrexate level needed for my treatment plan?
Questions people ask
Why does the time of my blood draw matter so much?
Methotrexate levels fall as your body clears the drug. A level that is expected 24 hours after a high-dose infusion would be well above target at 72 hours. That is why each result is read against the exact hours since your dose.
I take methotrexate once a week for arthritis or psoriasis. Do I need levels?
Usually not. For weekly low-dose treatment, clinicians follow blood counts, liver tests and kidney tests, and methotrexate levels aren't routine. Ask your clinician what is being monitored for you.
How do I convert between µmol/L and ng/mL?
Multiply µmol/L by about 454 to get ng/mL, so 1 µmol/L is about 454 ng/mL and 0.1 µmol/L is about 45 ng/mL. To go the other way, divide ng/mL by 454.
Show the science and sources
Units
- 1 µmol/L = 1 µmol/L
- 1 µmol/L = 1000 nmol/L
- 1 µmol/L = 454.4 ng/mL
- 1 µmol/L = 0.4544 µg/mL
Open the Methotrexate unit converter
For clinicians and researchers
Methotrexate is measured in serum or plasma by immunoassay or by chromatographic methods (HPLC, LC-MS/MS). Immunoassays cross-react with the inactive metabolite DAMPA and have a lower limit of quantitation of about 20 nmol/L, which limits use in low-dose treatment. After glucarpidase, DAMPA (half-life about 9 hours) can make immunoassays read falsely high for up to about 48 hours, so FDA labeling prefers a chromatographic method. The terminal half-life is about 3 to 10 hours with low-dose treatment and 8 to 15 hours after high-dose IV treatment, and it is prolonged by renal impairment and third-space fluid. FDA labeling describes glucarpidase for toxic plasma concentrations (above 1 µmol/L) with delayed clearance due to impaired renal function. Timing of measurements and rescue varies across regimens, so no single cut-off applies to every protocol. The 2018 consensus guideline lists the common high-dose regimens and notes that most hospitals keep monitoring until the level is 0.1 to 0.2 µmol/L or lower. Because 1 µmol/L is about 454 ng/mL (molecular weight 454.4), mass and molar results differ by a factor of about 454. Critical-limit practice varies: in a 2026 survey of 411 US hospitals, about 7% list methotrexate as a call-all-results test and about 15% give a numeric limit (median around 22 µmol/L, from very wide ranges). Polyglutamated forms inside red cells are a separate test.
LOINC 14836-1 (primary), 3793-7, 51600-5, 51611-2, 51602-1, 3795-2, 3796-0, 3797-8. Specimen: serum-or-plasma.
Sources
- Methotrexate Injection: prescribing information, U.S. Food and Drug Administration (DailyMed) (2026)
- Trexall (methotrexate) tablets: prescribing information, U.S. Food and Drug Administration (DailyMed) (2026)
- Voraxaze (glucarpidase) for injection: prescribing information, U.S. Food and Drug Administration (DailyMed) (2024)
- Consensus Guideline for Use of Glucarpidase in Patients with High-Dose Methotrexate Induced Acute Kidney Injury and Delayed Methotrexate Clearance, The Oncologist (2018)
- Drug Critical Limits for Urgent Physician Notification, Clinical Pharmacology & Therapeutics (2026)
- Therapeutic Drug Monitoring of Low Methotrexate Doses for Drug Exposure and Adherence Assessment: Pre-Analytical Variables, Bioanalytical Issues, and Current Clinical Applications, International Journal of Molecular Sciences (2024)
- Methotrexate (PubChem CID 126941), PubChem, National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.