A 52-year-old man with bipolar disorder has been on lithium 900 mg daily for 3 years with good mood stabilization. He presents to the psychiatry clinic for routine follow-up. Which investigation is most appropriate to assess his current lithium status and guide dosing adjustments?

## Lithium Monitoring: Serum Level Timing **Key Point:** Serum lithium levels must be drawn at a standardized time — 12 hours post-dose (or immediately before the next dose) — to ensure reproducibility and valid interpretation across visits. ### Why 12-Hour Post-Dose Timing? 1. **Steady-state kinetics:** Lithium reaches steady state in 5–7 days; at this point, the 12-hour level is a reliable proxy for total body burden and therapeutic efficacy. 2. **Therapeutic window:** The therapeutic range is 0.6–1.2 mEq/L (trough level); toxicity risk increases significantly above 1.5 mEq/L. 3. **Standardization:** Using a fixed time point (e.g., just before the morning dose if lithium is given twice daily) allows valid comparison between clinic visits and between patients. ### Comparison of Lithium Monitoring Approaches | Approach | Timing | Clinical Use | Validity | | --- | --- | --- | --- | | **Serum level 12 hrs post-dose** | Trough (immediately pre-next dose) | Routine monitoring, dose adjustment | Gold standard; reproducible | | **Urinary lithium excretion** | 24-hour collection | Assessment of renal handling; not routine | Indirect; rarely used in practice | | **2-hour post-dose level** | Peak level | Research only; not clinically used | Reflects absorption, not steady state | | **Random level, no timing** | Unspecified | None; invalid for clinical decisions | Uninterpretable; cannot guide dosing | **High-Yield:** The 12-hour post-dose (trough) level is the **only valid** serum lithium measurement for routine clinical monitoring and dosing decisions. This is mandated by all major psychiatric and pharmacological guidelines. **Clinical Pearl:** If a patient's level is drawn at an incorrect time (e.g., 2 hours post-dose), the result will be falsely elevated and may lead to unnecessary dose reduction. Always document the exact time of blood draw relative to the last lithium dose. **Mnemonic:** **TROUGH = TRUE** — Always draw lithium levels at the **trough** (12 hours post-dose) for **true** clinical interpretation.