Timing of Lithium Blood Level After Switching from Morning to Evening Dose
You should wait 5-7 days after switching lithium from a morning to evening dose before checking a blood level to ensure steady-state conditions are re-established. 1, 2
Rationale for the 5-7 Day Wait Period
The timing is based on lithium's pharmacokinetic properties and the need to reach steady-state after any dosing schedule change:
Lithium has an elimination half-life of 18-36 hours, meaning it takes approximately 5 half-lives (5-7 days) to reach a new steady-state after any change in dosing schedule 2
The FDA label explicitly states that serum levels should be drawn 8-12 hours after the previous dose when lithium concentrations are relatively stable, and this standardization is critical for accurate therapeutic monitoring 1
During the acute phase or after dosing changes, guidelines recommend checking serum concentrations twice per week until levels and clinical condition stabilize 3, 1
Standardized Blood Draw Timing
When you do check the level after the 5-7 day waiting period:
Draw the blood sample 12 hours after the evening dose (i.e., in the morning before any dose is given) to obtain the standardized 12-hour lithium concentration 1, 2, 4
This 12-hour standardization is essential because lithium reaches peak plasma concentration at 1-2 hours for standard-release forms and 4-5 hours for sustained-release forms, then gradually declines 2
The 12-hour standardized serum lithium concentration is considered the gold standard for monitoring, as it provides consistent, comparable values regardless of dosing schedule 4, 5
Clinical Monitoring During the Transition
While waiting the 5-7 days:
Monitor the patient clinically for signs of toxicity, as toxicity can occur at levels ≥1.5 mEq/L and serious toxicity at >2.0 mEq/L 3, 6
Total reliance must not be placed on serum levels alone; accurate patient evaluation requires both clinical and laboratory analysis 1
If any concerning symptoms develop during the transition period, check the level immediately rather than waiting the full 5-7 days 3
Important Caveat
Once-daily evening dosing may require maintaining serum concentrations in the upper therapeutic range (0.8-1.0 mmol/L) rather than 0.6-0.8 mmol/L, as the longer interval between doses can affect drug distribution 2