Timing of First Lithium Level After Initiation
Check the first lithium level 5-7 days after starting 450mg lithium nightly, as this is when steady-state concentrations are achieved. 1, 2
Rationale for 5-7 Day Timing
- Lithium requires approximately 5-7 half-lives (7-10 days) to reach steady-state serum concentrations, with an elimination half-life of 18-36 hours 1, 3
- The FDA label specifies that serum levels should be determined twice per week during the acute phase until both the serum level and clinical condition stabilize 2
- Checking levels before steady state (i.e., before day 5) provides unreliable information for dose adjustment, as concentrations are still rising 1
- Waiting beyond 7-8 days is unnecessary since steady state should be achieved, and delays appropriate dose adjustments 1
Proper Sample Collection Technique
- Draw blood samples 12 hours after the last dose for standard-release formulations to measure trough levels when lithium concentrations are relatively stable 2, 3, 4
- For once-daily evening dosing (which 450mg nocte represents), the 24-hour post-dose level should serve as the control value rather than the 12-hour level 2, 5
- Specifically for your patient on 450mg nightly: draw the level in the morning before the next evening dose (approximately 24 hours post-dose) 5
Expected Therapeutic Context
- A starting dose of 450mg daily is extremely low and typically produces levels of only 0.2-0.6 mEq/L 1
- This dose may be appropriate for elderly patients, those with renal impairment, or lithium augmentation strategies, but is subtherapeutic for acute bipolar disorder (which requires 1.0-1.5 mEq/L) 1, 2
- For acute mania, the FDA recommends 600mg three times daily (1800mg total) as the typical starting regimen 2
Ongoing Monitoring Schedule
- After the initial level at 5-7 days, continue checking levels twice weekly until stabilized 1, 2
- Once stable, transition to monitoring every 3-6 months during maintenance therapy 1, 6
- Increase monitoring frequency to every 1-2 weeks after dose adjustments, addition of interacting medications (NSAIDs, ACE inhibitors, ARBs, thiazides), changes in renal function, or significant weight changes 1
Critical Baseline and Concurrent Monitoring
- Before initiating lithium, obtain: complete blood count, thyroid function tests (TSH, free T4), renal function (BUN, creatinine, eGFR), serum electrolytes including calcium, and urinalysis 1, 7, 6
- Check comprehensive metabolic panel, electrolytes, BUN, and creatinine 2-4 weeks after starting lithium to capture early changes in renal function 1
- Monitor thyroid function, weight, and blood pressure annually during maintenance 1
Common Pitfalls to Avoid
- Do not check levels before day 5, as pre-steady-state levels are misleading and may prompt inappropriate dose increases 1
- Avoid drawing levels at random times—the 12-hour (or 24-hour for once-daily dosing) standardized timing is essential for interpretation 2, 3, 4, 5
- Do not rely solely on serum levels—accurate patient evaluation requires both clinical assessment and laboratory analysis 2
- Ensure the patient maintains adequate hydration and avoid concomitant NSAIDs, which significantly increase lithium levels and toxicity risk 1, 7