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Unexplained Lithium Level Drop — MRCPsych Paper B MCQ

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HardMood DisordersUnexplained Lithium Level DropMRCPsych Paper B

A 28-year-old woman with bipolar I disorder has been stable on unchanged doses and brands of lithium and carbamazepine. Her 12-hour trough lithium concentration was previously 0.7 mmol/L but is now 0.4 mmol/L. A correctly timed repeat sample one week later is also 0.4 mmol/L. Her renal function and electrolytes are unchanged, and she reports no change in habitual fluid intake, dietary sodium, caffeine consumption or other medication. Which is the most likely explanation?

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Correct answer: E — Intermittent non-adherence to lithium

The most likely explanation is intermittent non-adherence to lithium. Missed doses commonly produce subtherapeutic concentrations, and this should be assessed when an otherwise stable level falls. The reproducible, correctly timed 12-hour trough samples make sampling or isolated analytical error unlikely. Lithium is not hepatically metabolised; it is excreted almost entirely by the kidneys, so carbamazepine's hepatic enzyme induction cannot increase lithium metabolism or renal clearance. Carbamazepine may increase the risk of neurotoxicity when combined with lithium, including at therapeutic lithium concentrations, but it does not characteristically lower the lithium level. Increased fluid intake can contribute to a low concentration, but the stem explicitly excludes a change in fluid intake.

Reference: NHS Specialist Pharmacy Service. Supporting lithium safety across the system: Subtherapeutic level section. 2026. https://sps.nhs.uk/articles/supporting-lithium-safety-across-the-system/