Methotrexate Unit Converter
Methotrexate Unit Converter
Convert high-dose methotrexate levels between µg/mL and µmol/L, with the time-based thresholds that decide when leucovorin rescue can stop.
Methotrexate converter
Mass ⇄ molarMethotrexate 0.5 µg/mL at 48 hours
Formula and conversion factor
µg/mL = µmol/L ÷ 2.2005
- 2.2005
- derived from the molecular weight of methotrexate, 454.44 Da (1000 ÷ 454.44)
- mg/L
- numerically identical to µg/mL
- protocol thresholds
- typical published thresholds are shown below, but the specific high-dose protocol in use always governs over a generic figure
Worked example
Methotrexate 0.5 µg/mL at 48 hours
0.5 × 2.2005 = 1.100 µmol/L
= 0.5 mg/L
Typical thresholds after a high-dose infusion
| Time after infusion start | Level suggesting delayed clearance |
|---|---|
| 24 hours | > 10 µmol/L |
| 48 hours | > 1 µmol/L |
| 72 hours | > 0.1 µmol/L |
| Stop leucovorin | < 0.05 – 0.1 µmol/L |
Common causes of delayed clearance
| Mechanism | Examples |
|---|---|
| Renal impairment | Methotrexate is nephrotoxic and renally cleared — a vicious cycle |
| Third-space fluid | Pleural effusion or ascites acting as a slow-release reservoir |
| Interacting drugs | NSAIDs, proton pump inhibitors, penicillins, probenecid, trimethoprim |
A level that decides when rescue can stop
High-dose methotrexate is given with a rescue agent, leucovorin (folinic acid), and the level is monitored not simply to confirm the dose was correct but to decide when leucovorin can safely stop. Delayed clearance is life-threatening: methotrexate that lingers in the circulation causes severe myelosuppression, mucositis and acute kidney injury, and the level — not a fixed number of rescue doses — is what should drive how long rescue continues.
Typical thresholds quoted after a high-dose infusion are below 10 µmol/L at 24 hours, below 1 µmol/L at 48 hours and below 0.1 µmol/L at 72 hours, with leucovorin continued until the level falls to roughly 0.05 – 0.1 µmol/L. These figures vary between protocols, and the protocol actually in use always governs over any generic threshold — check the local regimen before acting on a result.
Delayed clearance has a short list of usual causes. Renal impairment is the commonest — methotrexate is itself nephrotoxic and is cleared renally, so it can impair its own clearance in a self-reinforcing cycle. A third-space fluid collection such as a pleural effusion or ascites can act as a slow-release reservoir, extending exposure long after the infusion has finished. Interacting drugs that reduce renal clearance are also a frequent cause — NSAIDs, proton pump inhibitors, penicillins, probenecid and trimethoprim all do this and are best avoided around a high-dose cycle. For severe delayed clearance with a rising creatinine, glucarpidase — an enzyme that rapidly inactivates circulating methotrexate — is available and should be considered without delay.
Most laboratories now report the result directly in µmol/L, which is treated as the primary unit here; mg/L and µg/mL are shown for reference for the minority of systems still reporting mass units.
Frequently asked questions
How do I convert methotrexate from µg/mL to µmol/L?
Multiply by 2.2005. A level of 0.5 µg/mL is 1.100 µmol/L. Most laboratories now report µmol/L directly, so this conversion is mainly needed for older literature and mass-unit reports.
What methotrexate level is safe at 48 hours?
A commonly quoted threshold is below 1 µmol/L at 48 hours after a high-dose infusion, but the specific protocol’s own thresholds always take precedence over a generic figure.
When can leucovorin rescue stop?
Once the methotrexate level falls to roughly 0.05 – 0.1 µmol/L, per the protocol in use. Stopping rescue on a fixed schedule rather than a level is unsafe if clearance is delayed.
What causes delayed methotrexate clearance?
Renal impairment, a third-space fluid collection such as a pleural effusion or ascites, and interacting drugs including NSAIDs, proton pump inhibitors, penicillins, probenecid and trimethoprim.
Related calculators
References
- Howard SC et al. Preventing and managing toxicities of high-dose methotrexate. Oncologist. 2016;21(12):1471–82.
- Widemann BC, Adamson PC. Understanding and managing methotrexate nephrotoxicity. Oncologist. 2006;11(6):694–703.