Phenytoin Dose From Two Steady-State Levels (Michaelis-Menten)
Phenytoin Km and Vmax from two steady-state levels on two different daily doses (Michaelis-Menten).
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- First daily dose
- 300 mg/day
- First dose form
- Sodium salt (capsules, injection)
- Steady-state level on the first dose
- 4.929 mcg/mL
- Second daily dose
- 400 mg/day
- Second dose form
- Sodium salt (capsules, injection)
- Steady-state level on the second dose
- 11.152 mcg/mL
- Target level
- 15 mcg/mL
Result: About 429.1 mg/day as the sodium salt (394.7 mg/day as phenytoin acid) for a level of 15 mcg/mL; Km 4.001 mcg/mL, Vmax 500 mg/day.
The tool opens with these values already filled in. Replace them with your own.
What you enter
- First daily dose
- First dose form: sodium salt (sodium) or free acid (acid)
- Steady-state level on the first dose
- Second daily dose
- Second dose form: sodium salt (sodium) or free acid (acid)
- Steady-state level on the second dose
- Target level
- With one level only: the Km to use (no default)
The other 2 fields
- A dose to test (optional)
- Form of the dose to test
How this is calculated
Citation
Ludden TM, Allen JP, Valutsky WA, et al. Individualization of phenytoin dosage regimens. Clin Pharmacol Ther. 1977;21(3):287-293 (Km and Vmax from two steady-state levels); Dilantin label for the 8% difference between the sodium salt and the free acid. Read the sources: Ludden 1977 ↗, Dilantin label ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.