When should a patient's lithium levels be drawn after their last dose?

Timing of Lithium Level Measurement

Lithium levels should be drawn 12 hours after the last dose for standard-release formulations and 24 hours after the last dose for once-daily extended-release formulations to obtain accurate trough levels for therapeutic monitoring. 1, 2

Standard Timing for Lithium Level Measurement

• For standard-release lithium formulations taken multiple times daily, blood samples should be drawn 12 hours after the last dose when lithium concentrations are relatively stable 1
• For once-daily extended-release formulations, blood samples should be drawn 24 hours after the last dose (immediately before the next dose) to obtain true trough levels 3
• The FDA label specifically states that "blood samples for serum lithium determination should be drawn immediately prior to the next dose when lithium concentrations are relatively stable (i.e., 8-12 hours after the previous dose)" 1

Rationale for Timing

• Lithium has a narrow therapeutic window, making precise monitoring essential to balance efficacy and safety 2
• Standard-release lithium reaches peak plasma concentration at 1-2 hours, while sustained-release forms peak at 4-5 hours after administration 2
• The elimination half-life of lithium is 18-36 hours, which means steady state is typically achieved after 5-7 days of consistent dosing 2
• Serum levels drawn at trough (immediately before next dose) provide the most consistent and comparable measurements for clinical decision-making 4

Frequency of Monitoring

• During acute treatment phase: Measure lithium levels twice per week until the serum level and clinical condition have stabilized 1
• For maintenance therapy: Monitor serum lithium levels at least every two months in uncomplicated cases 1
• For patients with kidney disease: More frequent monitoring is required, especially with GFR <60 ml/min/1.73 m² 5

Clinical Considerations

• The therapeutic range for lithium is 0.6-1.2 mEq/L for maintenance therapy and 1.0-1.5 mEq/L for acute mania 1
• Elderly patients often require lower doses and may exhibit signs of toxicity at serum levels normally tolerated by other patients 1
• Patients taking once-daily extended-release lithium have levels at 12 hours that are approximately 1.3 times higher than their 24-hour levels 3
• Using the wrong timing for blood draws may lead to inappropriate dosage adjustments and potential under-treatment or toxicity 3, 6

Common Pitfalls to Avoid

• Drawing levels at random times rather than at trough can lead to misleading results and inappropriate dosage adjustments 4
• Failing to standardize the timing of blood draws makes it difficult to compare results across time and with established therapeutic ranges 4
• Not accounting for the difference between standard-release and extended-release formulations when timing blood draws 3, 6
• Relying solely on serum levels without clinical assessment - the FDA label emphasizes that "total reliance must not be placed on serum levels alone" 1

Remember that lithium monitoring requires both accurate laboratory analysis and thorough clinical evaluation to ensure optimal therapeutic outcomes while minimizing toxicity risks.