Prediction Accuracy of Winter-Tozer Equations to Estimate Free Phenytoin Concentrations in Non-Critically Ill Hospitalized Patients

Consult Pharm. 2017 Aug 1;32(8):461-467. doi: 10.4140/TCP.n.2017.461.


Objective: To assess the accuracy of the standard and revised Winter-Tozer (WT) equations of predicting free phenytoin (PHT) concentrations in non-critically ill, hospitalized patients.

Design: Retrospective chart review.

Setting: Single-center, large urban community hospital.

Patients, participants: Patients with free PHT levels obtained during the study period were reviewed. Thirty-nine non-critically ill, hospitalized adults met the inclusion criteria. The study population consisted of 77% African-Americans and 77% individuals 65 years of age and older.

Interventions: PHT concentrations were measured at ambient temperature. Predicted free PHT concentrations were estimated as 10% of normalized total PHT concentrations using standard or revised WT equations in two groups of patients with hypoalbuminemia and end-stage renal disease (ESRD).

Main outcome measure(s): Correlation between measured versus predicted free PHT concentrations in patients with hypoalbuminemia or ESRD.

Results: Strong positive correlations were identified between measured and predicted free PHT concentrations for 27 patients with hypoalbuminemia (r [the Pearson correlation coefficient] = 0.96; P < 0.001) and 12 patients with ESRD (r = 0.95; P < 0.001).

Conclusion: This study population represented a largely understudied population in this research area. Use of traditional and revised WT equations in primarily elderly, non-critically ill African-American populations can be considered to accurately predict PHT levels in populations with altered PHT albumin binding resulting from hypoalbuminemia or ESRD and settings where free PHT levels are not readily obtainable.

MeSH terms

  • Adult
  • Aged
  • Aged, 80 and over
  • Anticonvulsants / administration & dosage
  • Anticonvulsants / pharmacokinetics*
  • Black or African American
  • Female
  • Hospitalization
  • Hospitals, Community
  • Humans
  • Hypoalbuminemia / complications*
  • Kidney Failure, Chronic / complications*
  • Male
  • Middle Aged
  • Phenytoin / administration & dosage
  • Phenytoin / pharmacokinetics*
  • Retrospective Studies
  • Young Adult


  • Anticonvulsants
  • Phenytoin