Phenytoin Correction for Hypoalbuminaemia (Sheiner–Tozer)
Estimates the albumin-corrected total phenytoin level using Sheiner–Tozer and its renal failure and critical illness variants.
Formula
Albumin must be in g/dL (divide g/L by 10 — handled automatically). The 0.25 variant is used in end-stage renal disease; 0.29 has been proposed in critical illness and the elderly.
Caveats & limitations
- All correction equations are estimates with wide error in critical illness, pregnancy, the elderly and uraemia — measure the free level when the result would change management.
- A 'normal' corrected level does not exclude toxicity, and vice versa — assess the patient (nystagmus, ataxia, encephalopathy).
- Phenytoin has non-linear (Michaelis–Menten) kinetics — dose changes produce disproportionate level changes.
- Valproate and other highly bound drugs displace phenytoin and further invalidate the correction.
Evidence & UK guidance
Primary evidence
Sheiner LB, Tozer TN. Clinical pharmacokinetics: the use of plasma concentrations of drugs. In: Clinical Pharmacology: Basic Principles in Therapeutics. New York: Macmillan; 1978.
Winter MG, Tozer TN. Phenytoin. In: Evans WE, Schentag JJ, Jusko WJ, eds. Applied Pharmacokinetics: Principles of Therapeutic Drug Monitoring. 3rd ed. Vancouver: Applied Therapeutics; 1992.
UK practice
The BNF quotes a total phenytoin therapeutic range of 10–20 mg/L. UK laboratories can measure unbound (free) phenytoin on request — this is preferred over any correction equation when hypoalbuminaemia, renal failure or critical illness makes binding unpredictable.
Official resources
Formula and citations last verified: 2026-08-03
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