Phenytoin is highly protein-bound (~90% to albumin). In hypoalbuminemia, total phenytoin levels underestimate the active free fraction. The Winter-Tozer equation corrects for this.
Formula
Corrected Phenytoin = Measured / (0.2 × Albumin g/dL + 0.1)
For dialysis patients: Corrected = Measured / (0.1 × Albumin + 0.1)
Therapeutic Range
- 10–20 mcg/mL (40–79 μmol/L) — Standard therapeutic range
- <10 mcg/mL — Subtherapeutic
- >20 mcg/mL — Potentially toxic (nystagmus, ataxia, sedation)
When to Correct
Correct phenytoin levels when:
- Albumin <3.5 g/dL
- Patient on dialysis
- Critically ill or burn patients (altered protein binding)
- Pregnancy (decreased albumin and altered clearance)
If available, free phenytoin levels are more accurate than calculated correction.