Corrected total phenytoin (Sheiner-Tozer)
Corrects a measured total phenytoin when albumin is abnormal and the free fraction was not measured.
When to use
Patient with abnormal albumin and no free phenytoin measurement available, where the total concentration may underestimate the active fraction.
Pearls and pitfalls
- The equation was described for use when a free phenytoin measurement is not available.
- It is biased and imprecise in critical care, head trauma and general neurology, and depends on albumin, assay temperature, renal function, age and concomitant drugs.
- Derivative equations with different albumin coefficients improved accuracy in critical care and in elderly patients, but still need validation.
- Measured concentration is entered in µg/mL and albumin in g/dL, as in the original publication.
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Formula
Corrected phenytoin (µg/mL) = measured total phenytoin (µg/mL) divided by (0.2 × albumin (g/dL) + 0.1).
Evidence
Systematic review of phenytoin correction equations, with the Sheiner-Tozer equation and its derivatives stated in the text.
References
- 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. doi:10.1177/1060028016628166
Frequently asked questions
What is Corrected total phenytoin (Sheiner-Tozer)?
Corrects a measured total phenytoin when albumin is abnormal and the free fraction was not measured. Patient with abnormal albumin and no free phenytoin measurement available, where the total concentration may underestimate the active fraction.
How is the result calculated?
Corrected phenytoin (µg/mL) = measured total phenytoin (µg/mL) divided by (0.2 × albumin (g/dL) + 0.1).
How should the result be interpreted?
Systematic review of phenytoin correction equations, with the Sheiner-Tozer equation and its derivatives stated in the text.
What are the sources?
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.