Timing of Trough Drug Level Sampling at Steady State

Understanding Steady State Requirements

For a patient at steady state, plasma samples to reflect trough drug levels should be obtained immediately before the next scheduled dose—typically 12-16 hours after the last dose for twice-daily regimens, or approximately 24 hours after the last dose for once-daily regimens 1, 2, 3
Before obtaining a trough level, the patient must have reached steady-state conditions, which occurs after approximately 5 drug half-lives following initiation of therapy or any dose change 1, 2, 3, 4
For most drugs, steady state is reached in 1 week (5-7 days) after stable daily dosing for drugs with typical half-lives 2, 3
For beta-lactam antibiotics with a loading dose, steady state is reached in 24-48 hours 5

For intermittent dosing, draw blood immediately before ingestion of the morning dose, which represents 12-16 hours after the last medication for twice-daily dosing 1, 2, 3, 4
For once-daily regimens, draw blood 24 hours after the last dose 1, 2, 3, 4
For continuous infusions, measure plasma steady-state concentrations rather than trough levels 5
For vancomycin (IV), obtain trough levels just before the fourth dose at steady state 6

Confirmation of steady-state achievement (≥5 half-lives at stable dosing) must be recorded 4

Sampling too soon after dose changes before steady state is reached produces misleading results 4
Drawing during the absorption phase (within 3-10 hours of oral dosing) yields falsely elevated and unpredictable concentrations 2, 3

Routine repeat monitoring is generally not necessary unless significant clinical changes occur, such as changes in circulatory, renal, or hepatic function 5
Repeat trough levels if new drug interactions are introduced that may alter metabolism 2, 3
Repeat trough levels if therapeutic interventions are implemented, such as fluid expansion, albumin, catecholamines, or renal replacement therapy 5
Repeat trough levels if clinical deterioration or suspected toxicity develops 4