From the Guidelines
A plasma sample to reflect the trough drug level in a patient at steady state should be obtained immediately before the next scheduled dose of medication, typically 12 to 16 hours after the last medication for most antidepressants. This recommendation is based on the guidelines provided by the AGNP-TDM Expert Group Consensus Guidelines, which suggest that blood should be collected for therapeutic drug monitoring (TDM) in steady-state conditions, at least 5 drug half-lives after changes in dose and during the terminal β-elimination phase 1.
Key Considerations
- The patient should have reached steady state, which is generally achieved after 1 week of stable daily dosing for most antidepressants, except for fluoxetine 1.
- The sampling time should be delayed after a modification of the dose or after prescription of a comedication that may inhibit or enhance the metabolism of the drug to be measured, until steady-state conditions are reached again 1.
- Accurate timing is essential because sampling too early or too late can lead to falsely high or low measurements, potentially resulting in inappropriate dose adjustments.
Clinical Implications
- The trough level is particularly important for medications with narrow therapeutic windows, where both efficacy and toxicity are closely tied to blood concentrations.
- TDM should be used to minimize adverse effects or optimize clinical efficacy, especially in patients who may particularly benefit from it, such as those with known plasma concentration ranges for clinical doses 1.
From the Research
Plasma Sample Collection for Trough Drug Level
To determine the trough drug level in a patient at steady state during a course of medication, it is essential to understand when to collect the plasma sample.
- The trough effect is generally referred to as the effect before the next dose is taken, usually the effect before the first morning dose 2.
- This means that the plasma sample should be obtained immediately before the next dose is administered, which is typically just before the first morning dose.
- The trough level is a critical measure, as it estimates the minimum drug concentration in the patient's system during a dosing interval.
- Maintaining a trough concentration above the minimum effective concentration is crucial for optimal therapy, as seen in the case of vancomycin therapy 3.
- The timing of plasma sample collection is vital, as the concentration of the drug in the blood can vary significantly depending on the sampling site and time 4.
Considerations for Steady State
- At steady state, the plasma concentrations within a dosing interval will increase until the time course of change in plasma concentrations becomes invariant from one dosing interval to the next 5.
- The effective rate of drug accumulation can be used to estimate the time to steady state, which is essential for determining when to collect plasma samples for trough level measurement 5.
- Understanding the pharmacokinetics of the drug, including its half-life and elimination rate, is crucial for determining the optimal timing of plasma sample collection 6.