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Corrected Phenytoin Level (Sheiner-Tozer / Winter-Tozer)

Corrected phenytoin = measured total level ÷ (0.2 × albumin g/dL + 0.1); with CrCl <10 mL/min or ESRD, ÷ (0.1 × albumin + 0.1). The therapeutic total range is 10–20 mg/L; a free phenytoin level is preferred when available.

  • Measured total phenytoin (mg/L or µg/mL or µmol/L)
  • Serum albumin (g/dL or g/L)
  • Renal function: CrCl ≥ 10 mL/min; CrCl < 10 mL/min / ESRD / dialysis
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Interpretation

ResultMeaning
< 10 mg/LSubtherapeutic
10–20 mg/LTherapeutic
> 20 mg/LToxic range (nystagmus >20, ataxia >30, lethargy >40)

Formula

Corrected = measured ÷ (0.2 × albumin + 0.1) · CrCl <10: measured ÷ (0.1 × albumin + 0.1) · albumin in g/dL

Clinical pearls

  • Only valid for TOTAL phenytoin levels; the equation tends to underestimate free levels in critically ill and elderly patients.
  • Free phenytoin (therapeutic 1–2 mg/L) is the gold standard when albumin is low, in renal failure, or with valproate co-therapy.
  • Fosphenytoin levels drawn too soon after a dose overestimate phenytoin.

Examples

corrected phenytoin level 8 albumin 2.0dilantin level 12 with albumin 25 g/L on dialysissheiner tozer phenytoin 15 alb 3.0

FAQ

Why correct phenytoin for albumin?

Phenytoin is about 90% protein bound. Low albumin raises the free (active) fraction, so a "normal" total level can be toxic.

What is the renal-failure version?

With creatinine clearance under 10 mL/min, uraemic toxins displace phenytoin further, so the albumin coefficient falls from 0.2 to 0.1.

Evidence

  1. Liponi DF, Winter ME, Tozer TN. Renal function and therapeutic concentrations of phenytoin. Neurology 1984;34:395–7.
  2. Anderson GD et al. Revised Winter-Tozer equation for normalized phenytoin concentrations in trauma and elderly patients with hypoalbuminemia. Ann Pharmacother 1997;31:279–84.

Pharmacology & Dosing · Reviewed 2026-10-14 · Methodology