Lithium Blood Levels Should Be Drawn 12 Hours After the Last Dose
For patients taking lithium, blood samples to check lithium levels should be drawn 12 hours after the last dose (or 8-12 hours, with 12 hours being optimal). This is the standard timing that correlates with the vast body of published lithium dosing data and therapeutic ranges 1.
Key Timing Considerations
Standard Immediate-Release Formulations
- Draw blood 12 hours after the last dose 1, 2
- This timing captures the trough level when lithium concentrations are relatively stable
- The FDA label explicitly states: "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
Once-Daily Extended-Release Formulations
There is conflicting evidence on this point:
- Some evidence suggests 24 hours for once-daily sustained-release preparations, as the 12-hour level may be 1.3 times higher than the true trough 3
- However, pharmacokinetic data shows that extended-release formulations are 90-100% absorbed by 4-8 hours, and blood levels at 12 hours are essentially identical to immediate-release formulations 4
- The most practical approach: Draw at 12 hours to compare with established therapeutic ranges, but be aware that for once-daily dosing, some clinicians prefer 24-hour levels 2
Clinical Reality: Compliance Is Poor
Recent real-world data reveals a significant problem:
- Approximately 45-50% of lithium blood tests do not comply with the 12-hour timing recommendation 5
- Blood tests taken >14 hours after dosing show significantly lower lithium levels (0.41 vs 0.64 mmol/L) 5
- This non-compliance can lead to inappropriate dose adjustments
Practical Algorithm
For twice or three-times daily dosing: Draw blood 12 hours after the last dose (typically morning draw before morning dose)
For once-daily evening dosing:
- Standard approach: Draw 12 hours after dose (morning draw)
- Alternative approach: Draw 24 hours after dose for true trough
- Choose one method and remain consistent for that patient
Document the actual time between last dose and blood draw, as this significantly affects interpretation 6, 5
If timing is off: A new equation (eLi12) can estimate the 12-hour level from blood drawn at other time points, showing 97% accuracy for samples taken 3-24 hours after dosing 6
Monitoring Frequency
- Acute phase: Twice weekly until stable 1
- Maintenance: Every 2 months minimum in uncomplicated cases 1
- Long-term: Every 3-6 months with renal and thyroid function 7
Critical Caveat
Never rely solely on serum levels - accurate patient evaluation requires both clinical assessment and laboratory analysis 1. Elderly patients and those sensitive to lithium may show toxicity at levels ordinarily tolerated by others (1.0-1.5 mEq/L) 1.