In adjusting phenytoin level for renal failure, the adjusted concentration is calculated by dividing the measured concentration by which expression?

• A. Measured concentration divided by [(0.1 × Albumin) + 0.1]
• B. Measured concentration divided by [(0.1 × Albumin) − 0.1]
• C. Measured concentration divided by [(0.2 × Albumin) + 0.1]
• D. Measured concentration divided by albumin alone

Answer: (A)

Phenytoin is highly albumin-bound, so low albumin (as can occur in renal failure) lowers the total measured concentration even though the free, active portion may be higher. To estimate the true level accounting for albumin binding, use the Sheiner-Tozer correction.

Corrected phenytoin concentration = measured phenytoin concentration ÷ (0.2 × albumin + 0.1), with albumin in g/dL. The 0.2 × albumin term reflects protein binding extent, and the +0.1 accounts for nonglobular binding factors.

For example, if the measured phenytoin is 12 mg/L and albumin is 2.5 g/dL, the denominator is 0.2 × 2.5 + 0.1 = 0.5 + 0.1 = 0.6. Corrected concentration = 12 / 0.6 = 20 mg/L.

Other forms (like using 0.1 × albumin, subtracting 0.1, or using albumin alone) do not align with this validated adjustment method.