When should a lithium serum level be drawn if the medication is taken at night?

Timing of Lithium Level Collection for Night-Time Dosing

If you take lithium at night, you should get your lithium level drawn 12 hours after your last dose—meaning the blood draw should occur in the morning, approximately 12 hours after your nighttime dose. 1

Standard Timing Protocol

The FDA explicitly states that blood samples for serum lithium determination should be drawn immediately prior to the next dose when lithium concentrations are relatively stable, specifically 8-12 hours after the previous dose. 1 This timing captures the "trough level"—the lowest concentration before the next dose—which is the standard reference point for therapeutic monitoring. 2, 3

• For patients taking lithium at night (e.g., 10 PM), the blood draw should ideally occur the following morning between 8-10 AM to achieve the 12-hour window. 1
• This 12-hour standardized timing is critical because therapeutic ranges (0.6-1.2 mEq/L for maintenance) are based on measurements taken at this specific interval. 1, 2

Special Consideration for Once-Daily Dosing

If you take lithium once daily at night, the blood draw should occur 24 hours after your dose—meaning the next evening before taking your next dose. 4

• Research demonstrates that for once-daily sustained-release preparations, measuring at 12 hours yields levels approximately 1.3 times higher than true trough levels measured at 24 hours. 4
• Using the 12-hour timing for once-daily dosing can result in falsely elevated readings, potentially leading to underdosing of patients. 4
• The appropriate trough level for once-daily dosing is measured at 24 hours post-dose to prevent patients from being maintained on lower-than-required doses. 4

Clinical Reality and Compliance Issues

Approximately 45-50% of lithium blood tests in real-world practice do not comply with the recommended 12-hour timing, with patients often presenting for blood draws anywhere from 0.5 to 120 hours after their last dose. 5 This non-compliance significantly affects accuracy:

• Blood tests taken more than 14-16 hours after the lithium dose show significantly lower lithium levels (0.41 vs. 0.64 mEq/L in one study). 5
• Recent research has developed an equation (eLi12) that can estimate the 12-hour level when blood is drawn at other time points, showing 97% accuracy for samples taken 3-24 hours post-dose. 6

Common Pitfalls to Avoid

• Do not draw levels at random times without accounting for the timing difference, as this leads to misinterpretation of therapeutic adequacy. 5
• Do not use 12-hour timing for once-daily dosing—this is a critical error that results in overestimation of trough levels. 4
• Do not rely solely on serum levels—the FDA emphasizes that accurate patient evaluation requires both clinical assessment and laboratory analysis. 1
• During the acute treatment phase, levels should be checked twice per week until stabilized, regardless of timing. 1, 7

Practical Algorithm

1. Divided dosing (2-3 times daily): Draw blood 12 hours after the last dose 1
2. Once-daily dosing: Draw blood 24 hours after the dose (immediately before the next dose) 4
3. Non-standard timing: If blood must be drawn at a different time, document the exact hours since last dose for potential correction using estimation equations 6