Corrected Phenytoin by Sheiner–Tozer Equation Calculator

Overview

Estimates an albumin-adjusted total phenytoin concentration, with an optional coefficient for severe renal impairment.

Clinical Significance

The equation estimates a corrected total concentration when protein binding is altered. Direct free phenytoin measurement is preferred when available, especially in critical illness, renal failure, pregnancy or interacting therapy.

When to Use

• Use when total phenytoin is being interpreted in hypoalbuminaemia.
• Use the renal coefficient only in accordance with the local therapeutic-drug-monitoring protocol.
• Confirm sampling time, adherence, dose history and interacting medicines.

How It Is Calculated

• Corrected phenytoin = measured total concentration ÷ [(coefficient × albumin) + 0.1], using 0.275 conventionally or 0.2 in severe renal impairment where locally adopted.
• The calculator validates the required inputs before applying the equation and rounds only the displayed result.

Interpretation

• The equation estimates a corrected total concentration when protein binding is altered. Direct free phenytoin measurement is preferred when available, especially in critical illness, renal failure, pregnancy or interacting therapy.
• Review the component values and assumptions as well as the final result.

Worked Example

Enter clinically plausible values in the stated units. MedicalC applies: Corrected phenytoin = measured total concentration ÷ [(coefficient × albumin) + 0.1], using 0.275 conventionally or 0.2 in severe renal impairment where locally adopted. Review the displayed result against the source data and the relevant clinical pathway.

Patient Considerations

• Confirm that the patient and clinical setting match the population or physiologic assumptions behind the equation.
• Consider whether treatment, organ function, pregnancy, body composition, sampling conditions or acute illness materially alter interpretation.
• Use serial calculations only when methods and units remain comparable.

Limitations

• The correction can misclassify exposure in critically ill patients.
• It does not account fully for uraemia, competing protein-bound drugs or altered binding states.
• Direct free phenytoin measurement is more reliable when available.

Clinical Pearls

• Do not adjust a concentration without checking the sample timing.
• Review clinical toxicity and seizure control alongside the number.
• Use the laboratory's own equation if it differs from this implementation.

Common Mistakes

• Using albumin in g/L rather than g/dL.
• Automatically applying the renal coefficient without meeting local criteria.
• Changing a dose solely from a corrected estimate.

Evidence Base

The implementation uses the published or established relationship: Corrected phenytoin = measured total concentration ÷ [(coefficient × albumin) + 0.1], using 0.275 conventionally or 0.2 in severe renal impairment where locally adopted. The cited source should be reviewed alongside current specialty guidance before clinical deployment.

References

1. Wilfred PM, et al. Estimation of free phenytoin concentration in critically ill patients with hypoalbuminemia. Indian J Crit Care Med. 2022. — Primary publication, authoritative guideline or recognised clinical source supporting the calculation.