Albumin-Corrected Phenytoin

What the total level would read if albumin were normal, and why renal failure changes the arithmetic · v1.0

  • Enter the total phenytoin level and the serum albumin, and tick severe renal impairment or dialysis where it applies.
  • You get the level the total would read at a normal albumin, by the Sheiner-Tozer equation, banded against the usual 10 to 20 mcg/mL range.
  • Renal impairment switches the equation, because uraemia displaces phenytoin from albumin independently of the albumin concentration itself.
  • A total level that looks low or normal can sit alongside a free level that is already toxic, which is what the correction exists to show.
  • Read it as an estimate. Where toxicity is genuinely in question and a free phenytoin assay is available, send one.

  • Children and adolescents under 18.
  • A measured free phenytoin level, which the equation estimates rather than replaces.
  • The diagnosis of phenytoin toxicity, which rests on nystagmus, ataxia, dysarthria and lethargy read alongside the level.
  • A dose, a loading regimen or an infusion rate.
  • Any equation other than Sheiner-Tozer. Revised coefficients exist and perform better in some populations, and none has been shown best across all of them.

1. Measured Level and Albumin

2. Renal Function

Uraemia further reduces phenytoin's binding to albumin, independent of the albumin level itself. Ticking this changes which correction equation is used. See Renal Staging if the creatinine clearance is not already known.

Clinical Disclaimer: This is an estimate, not a measured free level. Where toxicity is genuinely in question and a free phenytoin assay is available, a directly measured free level is more reliable than any corrected estimate.

1. Why This Matters at the Bedside

Phenytoin is about 90 per cent protein bound at a normal albumin, and the 10 to 20 mcg/mL total range most laboratories quote as therapeutic assumes that binding. In hypoalbuminaemia a smaller fraction is bound.

  • A total level of 8 mcg/mL in a patient with an albumin of 2.0 g/dL can represent the same free, active drug concentration as a total of 15 to 16 mcg/mL in a patient with a normal albumin.
  • Reading the total alone risks two opposite errors: dismissing a toxic free level as subtherapeutic, and pushing the dose up in response to a low total that is not low once corrected.

2. Who This Applies To

Hypoalbuminaemia significant enough to matter here is common in exactly the patients most likely to be on phenytoin for an acute indication: the critically ill, the malnourished, nephrotic syndrome, cirrhosis, and pregnancy. A dose adjusted from an uncorrected total level in any of these patients is being adjusted from the wrong number.

3. Two Equations, Not One

The original Sheiner-Tozer equation assumes normal renal function. Uraemia displaces phenytoin from albumin independently of the albumin concentration itself, which is why end-stage renal disease and dialysis use a separate equation with a smaller coefficient, derived by Liponi, Winter and Tozer in 1984.

  • Using the normal-renal equation in a dialysis patient understates the correction and can read a genuinely toxic free level as safe.
  • The renal equation is the better of the two available here, and it is still not a good one. Soriano and colleagues compared it against measured free levels in patients with end-stage renal disease on haemodialysis and found a percentage error of 75, with two thirds of samples out by more than half. Read the renal result as a prompt to send a free level, not as a substitute for one.

4. Where the Coefficients Come From, and Where They Do Not Agree

This tool uses the coefficients most widely taught and most widely reproduced, 0.2 for normal renal function and 0.1 for renal failure, both against a fixed 0.1 term. A 2016 systematic review found that revised coefficients, derived from larger and more contemporary patient samples, predicted measured free levels more accurately in some populations than the original equation did. There is no single equation shown to be best across every population, including critical illness, and the difference between equations matters most exactly when the correction matters most, at the extremes of albumin. Treat the corrected value here as one estimate among a documented family of them, not as a measured result.

5. A Level Does Not Diagnose Toxicity, and a Level Does Not Rule It Out

Nystagmus, ataxia, dysarthria, lethargy and, at the extreme, coma are the classical progression of phenytoin toxicity, but the level at which each appears varies between patients. A corrected level inside the reference range does not exclude toxicity in a symptomatic patient, and a level above range in an asymptomatic patient is not, on its own, a reason to withhold a needed dose. Correlate with the examination.

Abbreviations CrCl (Creatinine Clearance) · ESRD (End-Stage Renal Disease)
References
  1. Sheiner LB, Tozer TN. Clinical pharmacokinetics: the use of plasma concentrations of drugs. In: Melmon KL, Morrelli HF, eds. Clinical Pharmacology: Basic Principles in Therapeutics. 2nd ed. New York: Macmillan; 1978:71-72.
  2. Liponi DF, Winter ME, Tozer TN. Renal function and therapeutic concentrations of phenytoin. Neurology. 1984;34(3):395-397.
  3. Winter ME. Basic Clinical Pharmacokinetics. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2010.
  4. Kiang TKL, Ensom MHH. A comprehensive review on the predictive performance of the Sheiner-Tozer and derivative equations for the correction of phenytoin concentrations. Ann Pharmacother. 2016;50(4):311-325.
  5. Soriano VV, Tesoro EP, Kane SP. Characterization of free phenytoin concentrations in end-stage renal disease using the Winter-Tozer equation. Ann Pharmacother. 2017;51(8):669-674.
  6. Cheng W, Kiang TKL, Bring P, Ensom MHH. Predictive performance of the Winter-Tozer and derivative equations for estimating free phenytoin concentration. Can J Hosp Pharm. 2016;69(4):269-279.
  7. Pandey S, Sharma PK, Garg RK, Takalkar K. Unexplained encephalopathy with phenytoin toxicity: hyperammonemia, the underlying cause. Neurol India. 2018;66(6):1829-1831.
  8. Winter ME, Tozer TN. Phenytoin. In: Evans WE, Schentag JJ, Jusko WJ, eds. Applied Pharmacokinetics: Principles of Therapeutic Drug Monitoring. 3rd ed. Vancouver, WA: Applied Therapeutics; 1992.
How to Cite This Tool

DOIhttps://doi.org/10.5281/zenodo.22401624

AMA Style:Umakanth S. Albumin-Corrected Phenytoin. Version 1.0. MEDiscuss Clinical Decision Support System. Published 2026. Accessed . https://mediscuss.org/cdss/phenytoin-albumin-correction. doi:10.5281/zenodo.22401624

Vancouver Style:Umakanth S. Albumin-Corrected Phenytoin [Internet]. Version 1.0. MEDiscuss.org; 2026 [cited ]. Available from: https://mediscuss.org/cdss/phenytoin-albumin-correction. doi:10.5281/zenodo.22401624

Disclaimer and scope
Disclaimer: This decision-support system is written for healthcare professionals. It does not substitute clinical judgment, and the treating physician retains absolute responsibility for every diagnostic, dosing and therapeutic decision. The algorithms are built on the evidence as it stands, and the evidence moves. Verify the output against your current institutional protocol, and against the patient in front of you.
© 2026 MEDiscuss.org. All rights reserved. Provided free of charge for clinical use. Unauthorised reproduction or redistribution is strictly prohibited.
Category Foundational CalculatorsCalculator
Specialties Internal Medicine, Neurology, Critical Care

Written and maintained by

Dr Shashikiran Umakanth

Last revised 24 August 2026

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