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Unexplained Low Lithium — MRCPsych Paper B MCQ

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HardMood DisordersUnexplained Low LithiumMRCPsych Paper B

A 38-year-old woman with bipolar I disorder has taken the same brand and prescribed dose of modified-release lithium carbonate for 1 year. Her correctly timed 12-hour plasma lithium concentrations have consistently been 0.7–0.8 mmol/L. Carbamazepine was started 3 weeks ago for trigeminal neuralgia. Her latest 12-hour lithium concentration is 0.25 mmol/L, and a repeat correctly timed sample is 0.24 mmol/L. Her lithium dose and formulation are unchanged. Renal function and serum sodium are stable, and she reports no gastrointestinal symptoms or changes in salt, fluid or caffeine intake. What is the most likely explanation for the low lithium concentrations?

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Correct answer: A — Intermittent omission of lithium doses

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

B is correct. After two correctly timed, concordant low results, intermittent omission of lithium doses is the most likely explanation and should be explored non-judgementally. Carbamazepine is a strong hepatic enzyme inducer, but lithium is excreted almost exclusively by the kidneys rather than cleared through CYP metabolism. The lithium SmPC specifically places carbamazepine among interactions that may produce neurotoxicity without necessarily altering the lithium concentration. A laboratory error is unlikely after a similar repeat result. Malabsorption would require supporting gastrointestinal disease, surgery or symptoms, none of which are present. A major change in fluid intake can affect lithium concentrations, but this is explicitly excluded. The carbamazepine remains clinically relevant because combined treatment can cause neurotoxicity even at therapeutic lithium levels.

Reference: Electronic Medicines Compendium. Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, sections 4.5 and 5.2. Updated 21 January 2025. https://www.medicines.org.uk/emc/product/10828/smpc