Phenytoin level
Drug LevelsNormal range
10–20 µg/mL (total; correct for low albumin)
SI: 40–79 µmol/L
Critical high: > 30 µg/mL
☎ When to call
Ataxia or confusion on phenytoin, or level > 25 — hold and call.
What you do
- Toxicity: nystagmus → ataxia → confusion (in that order as levels climb)
- Low albumin? Ask for a free phenytoin level or corrected calculation
- Tube-fed patient: hold feeds before/after PO phenytoin per policy — this interaction is real
Causes of HIGH phenytoin level
Causes of LOW phenytoin level
Source: Pagana, Mosby's Manual of Diagnostic and Laboratory Tests, 7th ed.; Lehne's PharmacologyReference only — verify against your facility's policy.
