Lithium monitoring — MRCPsych Paper A MCQ

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EasyPsychopharmacologyLithium monitoringMRCPsych Paper A

A 42-year-old man with bipolar disorder is stable on lithium and attends for routine follow-up. He asks why blood tests are required when he feels well. What is the most likely explanation?

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Correct answer: E — Lithium has a narrow therapeutic index

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

E is correct. Lithium has a narrow therapeutic window: concentrations required for efficacy are relatively close to those associated with toxicity, and levels may rise with renal impairment, intercurrent illness, altered fluid or sodium intake, or interacting medicines. Regular plasma lithium measurements are therefore required even when the patient feels well; renal and thyroid function also require monitoring during long-term treatment. Lithium is not rapidly metabolised by the liver and is excreted almost exclusively by the kidneys, making A incorrect. It is not managed as a CYP3A4 inducer, so B is incorrect. Agranulocytosis is not a characteristic dose-related lithium toxicity; leukocytosis may occur, making C incorrect. Valproate-related displacement from albumin does not explain routine lithium monitoring, so D is incorrect.

Reference: electronic Medicines Compendium, Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, sections 4.2, 4.4 and 5.2, updated 21 January 2025. https://www.medicines.org.uk/emc/product/10828/smpc