How to Convert Phenytoin µg/mL to µmol/L
Phenytoin levels are reported in µg/mL (mg/L) in the US and in µmol/L in many other countries. The conversion factor is 3.96, from phenytoin's molecular weight of 252.27 g/mol.
Phenytoin µmol/L = phenytoin µg/mL × 3.96 Phenytoin µg/mL = phenytoin µmol/L ÷ 3.96 Phenytoin µg/mL to µmol/L Chart
| µg/mL | µmol/L | Context |
|---|---|---|
| 5 µg/mL | 19.8 µmol/L | Sub-therapeutic |
| 10 µg/mL | 39.6 µmol/L | Therapeutic (lower) |
| 15 µg/mL | 59.4 µmol/L | Therapeutic (mid) |
| 20 µg/mL | 79.2 µmol/L | Therapeutic (upper) |
| 25 µg/mL | 99.0 µmol/L | Toxic range |
| 30 µg/mL | 118.8 µmol/L | Toxic |
µmol/L to µg/mL Reverse Chart
| µmol/L | µg/mL |
|---|---|
| 40 µmol/L | 10.1 µg/mL |
| 60 µmol/L | 15.2 µg/mL |
| 79 µmol/L | 19.9 µg/mL |
| 100 µmol/L | 25.3 µg/mL |
Phenytoin Therapeutic and Toxic Ranges
| Measure | µg/mL | µmol/L |
|---|---|---|
| Total phenytoin (therapeutic) | 10–20 µg/mL | 40–79 µmol/L |
| Free phenytoin (therapeutic) | 1–2.5 µg/mL | ~4–10 µmol/L |
| Toxic | Above 20 µg/mL | Above 79 µmol/L |
Total vs Free Phenytoin and Low Albumin
This is the nuance that matters most. Phenytoin is about 90% bound to albumin, and only the free (unbound) fraction is active. When albumin is low — or in kidney disease and pregnancy — a total phenytoin level can look falsely low even when the free level is adequate. In those settings a measured free phenytoin (target ~1–2.5 µg/mL) is more reliable.
Corrected = measured ÷ (0.2 × albumin g/dL + 0.1) The corrected value estimates the level at normal albumin; a measured free phenytoin is preferred when it is available.
Worked Example
Suppose a report shows 79 µmol/L and you want µg/mL:
79 ÷ 3.96 = 19.9 µg/mL 19.9 µg/mL is at the top of the therapeutic range. Converting the other way, 10 µg/mL is about 39.6 µmol/L.
Frequently Asked Questions
How do you convert phenytoin µg/mL to µmol/L?
Multiply µg/mL by 3.96. For example, 15 µg/mL is about 59.4 µmol/L.
What is the therapeutic range?
Total 10–20 µg/mL (40–79 µmol/L); free 1–2.5 µg/mL.
Why measure free phenytoin?
Only free drug is active; total can mislead when albumin is low.
What are toxicity signs?
Nystagmus, ataxia, slurred speech and drowsiness above the therapeutic range.
References
- Medscape — Phenytoin Level: Reference Range and Interpretation.
- Phenytoin factor 3.96 (MW 252.27 g/mol); Sheiner-Tozer albumin-correction equation (standard therapeutic drug monitoring references).
Conclusion
Converting phenytoin between µg/mL and µmol/L is a ×3.96. Just remember the total level only tells part of the story — when albumin is low, the free phenytoin is what counts. Interpret results with your clinician.