Lithium Monitoring — MRCPsych Paper B MCQ

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ModerateMood DisordersLithium MonitoringMRCPsych Paper B

A 45-year-old woman with bipolar disorder has taken lithium for 5 years. Her dose and mood are stable, adherence is good, and her most recent plasma lithium level was 0.7 mmol/L. Renal and thyroid function and serum calcium are normal, and she takes no medicines that interact with lithium. According to NICE CG185, how frequently should her plasma lithium level routinely be measured?

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Correct answer: E — Every 6 months

The correct answer is E: every 6 months. NICE CG185 recommends measuring plasma lithium levels every 3 months during the first year and every 6 months thereafter in patients without additional risk factors. Three-monthly monitoring is required after the first year for older people and for those taking interacting medicines, at risk of renal, thyroid or calcium-related complications, with poor symptom control or adherence, or whose most recent lithium level was 0.8 mmol/L or higher. None applies here. Monthly monitoring is not routine for stable long-term treatment, while annual or symptom-triggered testing is too infrequent because lithium has a narrow therapeutic index and toxicity may develop without a preceding change in psychiatric symptoms.

Reference: National Institute for Health and Care Excellence. Bipolar disorder: assessment and management (CG185), section 1.10, recommendation 1.10.20. 2014, updated 2025. https://www.nice.org.uk/guidance/cg185/chapter/Recommendations