← Back to All Calculators
Shortcode 107 — Laboratory
Phenytoin Level Adjustment Calculator
Applies Winter-Tozer correction for hypoalbuminemia and renal dysfunction. Estimates albumin-adjusted total and free phenytoin levels with toxicity interpretation.
Patient Inputs
Reported lab value — mcg/mL
Enter a valid level (0.1–100 mcg/mL)
g/dL — normal 3.5–5.0; correction needed if ≠4.0
Enter a valid albumin (0.5–6.0 g/dL)
Affects protein binding and Winter-Tozer formula
Phenytoin Level Results
—
Reported Level
—
mcg/mL (total)
Therapeutic: 10–20 mcg/mL
—
Adjusted Level
—
mcg/mL (corrected)
Therapeutic: 10–20 mcg/mL
—
Est. Free Level
—
mcg/mL (unbound)
Therapeutic: 1.0–2.0 mcg/mL
—
Clinical Recommendation
—
Toxicity Thresholds
Adjusted Level >20 mcg/mL
Nystagmus risk — supratherapeutic. Dose reduction likely warranted.
Adjusted Level >30 mcg/mL
Ataxia, diplopia, slurred speech — significant toxicity. Hold doses; recheck level.
Adjusted Level >40 mcg/mL
Altered consciousness, paradoxical seizures — severe toxicity. Immediate medical attention required.
Free Level >2.0 mcg/mL
Free fraction supratherapeutic — even if total level appears normal. Monitor for adverse effects.
Effect of Hypoalbuminemia on Phenytoin Binding
| Albumin (g/dL) | Measured (mcg/mL) | Adjusted — Normal Renal | Adjusted — CKD | Clinical Implication |
|---|---|---|---|---|
| 4.0 (normal) | 15 | 15.0 | 15.0 | No correction needed |
| 3.0 | 15 | 21.4 | 30.0 | True level higher than measured |
| 2.5 | 15 | 25.0 | 37.5 | Supratherapeutic despite normal-looking level |
| 2.0 | 15 | 30.0 | 50.0 | Potentially toxic — correction critical |
| 1.5 | 15 | 37.5 | 66.7 | Severely toxic — free level markedly elevated |
Winter-Tozer Correction Formulas
Normal Renal Function: Adjusted Level = Measured / [(0.2 x Albumin) + 0.1] Severe CKD (eGFR <20) or Dialysis: Adjusted Level = Measured / [(0.1 x Albumin) + 0.1] Estimated Free Level: Free = Adjusted Level x 0.1 Therapeutic Ranges: Total (adjusted): 10-20 mcg/mL Free: 1.0-2.0 mcg/mL Note: In normal renal function, phenytoin is ~90% protein-bound. In severe CKD, uremic toxins compete for albumin binding sites, reducing the denominator coefficient from 0.2 to 0.1.
Medical Disclaimer
Phenytoin pharmacokinetics are complex and non-linear (zero-order kinetics at therapeutic concentrations). The Winter-Tozer equation provides an estimate only — free phenytoin measurement is the gold standard when available. Clinical correlation is essential. Dosing adjustments should involve a clinical pharmacist or neurologist. This tool does not replace professional clinical judgment.
