Phenytoin – Chronic Side Effects — SCE Neurology MCQ

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HardEpilepsy & Seizure DisordersPhenytoin – Chronic Side EffectsSCE Neurology

A 70-kg adult taking phenytoin remains in convulsive status epilepticus. A promptly available total phenytoin concentration is 8 mg/L and albumin is normal. Using the NHS Highland top-up formula with a target of 20 mg/L, what dose is required?

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Reveal the answer and explanation

Correct answer: D — 588 mg phenytoin sodium

Explanation lettering: B = shown as A · E = shown as B · A = shown as C · C = shown as D · D = shown as E

The formula is (20 − measured concentration) × 0.7 × weight. A uses the concentration deficit without the full distribution factor. B corresponds to 8 × 70. C is correct: (20 − 8) × 0.7 × 70 = 588 mg, rounded to a practicable dose under local pharmacy guidance. D omits the 0.7 factor. E applies 20 × 0.7 × 70 as if no phenytoin were present.

Reference: NHS Highland: phenytoin dose calculations: https://www.rightdecisions.scot.nhs.uk/tam-treatments-and-medicines-nhs-highland/adult-therapeutic-guidelines/nervous-system/seizures-guidelines/phenytoin-dose-calculations-guidelines/