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Phenytoin Pharmacokinetics – Hypoalbuminaemia — SCE Neurology MCQ

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HardEpilepsy & Seizure DisordersPhenytoin Pharmacokinetics – HypoalbuminaemiaSCE Neurology

A 70-year-old man with epilepsy has a breakthrough tonic-clonic seizure. Blood results show a phenytoin level of 8 mg/L (therapeutic range 10–20 mg/L) and albumin of 22 g/L. What must be considered when interpreting this phenytoin level?

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Correct answer: A — The corrected (free) phenytoin level may be within the therapeutic range due to hypoalbuminaemia

Phenytoin is highly protein-bound (~90%) to albumin. In hypoalbuminaemia, total phenytoin levels underestimate the free (active) fraction. The corrected phenytoin level can be calculated using the Sheiner-Tozer equation: Corrected level = Measured level / (0.2 × albumin [g/dL] + 0.1). With albumin 22 g/L (2.2 g/dL): Corrected = 8 / (0.2 × 2.2 + 0.1) = 8 / 0.54 = ~14.8 mg/L — within the therapeutic range. A: Increasing the dose risks toxicity. C: The level does not confirm non-adherence. D: The level is not toxic. E: Albumin significantly affects total phenytoin levels.

Reference: BNF; NICE NG217 (2025)