Phenytoin µg/mL → µmol/L
Convert phenytoin from µg/mL to µmol/L
Sources last checked: 2026-11-03
Phenytoin µg/mL → µmol/L
Convert phenytoin from µg/mL to µmol/L
Converts units only; reference ranges depend on the laboratory, the assay and the patient.
What is the Phenytoin µg/mL → µmol/L?
Phenytoin levels are reported in µg/mL (identical to mg/L) by most laboratories in Germany, Austria, Turkey and the USA, and in µmol/L by some laboratories that apply SI units strictly. This converter multiplies by 3.964, the factor that follows from the molar mass of phenytoin (252.27 g/mol). It converts the measured concentration, usually total phenytoin.
When to use it
- Comparing a phenytoin level in µg/mL with a guideline, study or letter in µmol/L
- Documenting therapeutic drug monitoring for a patient moving between laboratories
- Pharmacokinetic exercises using SI units
- Exam questions on antiepileptic drug monitoring
When not to use it
- Fosphenytoin doses, which are expressed in phenytoin sodium equivalents and are not concentrations
- Correcting a total level for low albumin: unit conversion does not do this, and correction formulas are only approximations
- Samples taken shortly after an intravenous fosphenytoin dose, when some immunoassays may still detect unconverted prodrug
How it is calculated
Phenytoin (µmol/L) = phenytoin (µg/mL) × 3.964
Clinical pearls and limitations
- Converts units only; reference ranges depend on the laboratory, the assay and the patient.
- 1 µg/mL = 1 mg/L = 3.964 µmol/L; conversely 1 µmol/L = 0.2523 µg/mL.
- Phenytoin is highly protein-bound. Low albumin, uraemia, late pregnancy and displacing drugs such as valproate raise the free fraction, so a 'normal' total level can hide a high free level; a free phenytoin level is more informative in these situations.
- Elimination is saturable (Michaelis-Menten kinetics): near the upper part of the usual dose range, a small dose increase can cause a disproportionate rise in the level.
- Interpret trough levels at steady state, which can take longer to reach than the half-life suggests because of the non-linear kinetics.
Frequently asked questions
How do I convert phenytoin from µg/mL to µmol/L?
Multiply by 3.964. For example, 14.5 µg/mL × 3.964 = 57 µmol/L.
Why is the phenytoin factor 3.964?
Phenytoin has a molar mass of 252.27 g/mol. 1 µg/mL equals 1 mg/L, and 1000 ÷ 252.27 = 3.964 µmol/L.
Does this converter work for free phenytoin?
Yes, the unit factor is the same for free and total phenytoin. The two have very different reference ranges, though, so always check which one was measured.
Evidence and guidelines
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<p><a href="https://medelix.ai/en/calculators/phenytoin-ug-ml-to-umol-l">Phenytoin µg/mL → µmol/L calculator by Medelix</a></p>For healthcare professionals and medical students. These tools support, and do not replace, clinical judgement. Always check results against the original source and your local protocols.