Lithium Level Timing — MRCPsych Paper B MCQ

Correct answer: A — Overestimate the standard 12-hour monitoring level

The correct answer is A. Routine lithium concentrations are standardised to a sample taken 12 hours after the last dose. A sample taken at 6 hours is closer to the post-dose peak and precedes further distribution and renal elimination, so it will generally be higher than the corresponding 12-hour concentration. Interpreting it against a standard 12-hour therapeutic range could therefore prompt an inappropriate dose reduction; a correctly timed sample should normally be obtained before routine dose adjustment. It is not the recommended peak measurement, so A is incorrect. C predicts the wrong direction, while D ignores the clinically important effect of sampling time. The 12-hour convention is used with both conventional and modified-release lithium, so E is also incorrect. The 12-hour value is best described as a standardised monitoring concentration rather than necessarily the literal trough for every dosing regimen.

Reference: Electronic Medicines Compendium, Lithium Carbonate Essential Pharma 250 mg film-coated tablets, SmPC sections 4.2, 4.4 and 5.2, updated 2025. https://www.medicines.org.uk/emc/product/10828/smpc