Phenytoin µmol/L → µg/mL
Convert phenytoin from µmol/L to µg/mL
Sources last checked: 2026-11-03
Phenytoin µmol/L → µg/mL
Convert phenytoin from µmol/L to µg/mL
Converts units only; reference ranges depend on the laboratory, the assay and the patient.
What is the Phenytoin µmol/L → µg/mL?
This converter turns a phenytoin concentration from µmol/L, used by some SI-reporting laboratories and in parts of the literature, into µg/mL (identical to mg/L), the unit most laboratories and dosing references use. It divides by 3.964 (equivalent to multiplying by about 0.2523), derived from the molar mass of phenytoin, 252.27 g/mol.
When to use it
- Reading a phenytoin level in µmol/L from an SI-reporting letter or study
- Using dosing nomograms or pharmacokinetic equations written in mg/L
- Comparing serial levels from laboratories that report in different units
When not to use it
- Interpreting a total level in hypoalbuminaemia, renal failure or with displacing drugs without considering the free fraction
- Interpreting the value: target ranges depend on the laboratory, whether total or free phenytoin was measured, and the clinical situation
How it is calculated
Phenytoin (µg/mL) = phenytoin (µmol/L) ÷ 3.964
Clinical pearls and limitations
- Converts units only; reference ranges depend on the laboratory, the assay and the patient.
- Dividing by 3.964 is the same as multiplying by 0.2523; dividing by 4 gives a quick estimate.
- µg/mL and mg/L are numerically identical, so a result in mg/L can be compared directly.
- Albumin-corrected phenytoin formulas are estimates and can be inaccurate in critically ill patients; a measured free level is preferred where available.
- Many drugs induce or inhibit phenytoin metabolism, and phenytoin itself induces other drugs; an unexpected level change often has a pharmacokinetic explanation.
Frequently asked questions
How do I convert phenytoin from µmol/L to µg/mL?
Divide by 3.964. For example, 33 µmol/L ÷ 3.964 = 8.3 µg/mL.
Is µg/mL the same as mg/L for phenytoin?
Yes. 1 µg/mL = 1 mg/L exactly, so no further conversion is needed between those two units.
Why can a phenytoin level be misleading when albumin is low?
Most laboratories measure total phenytoin. When albumin falls, a larger share circulates unbound, so the pharmacologically active free concentration can be high even though the total looks unremarkable.
Evidence and guidelines
Have a follow-up question?
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<p><a href="https://medelix.ai/en/calculators/phenytoin-umol-l-to-ug-ml">Phenytoin µmol/L → µg/mL 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.