As the prescribed drug is used or metabolized by the body, the drug level decreases. The lowest level of the drug in the patient's body is called the trough level. The peak for a drug is when the level of the drug in the patient's body is the highest.
To assess drug concentrations during the trough phase, blood should be drawn immediately before the next dose.
To assess peak levels, the time for drawing depends on the route of administration:
- Intravenous (IV): 15 - 30 minutes after injection/infusion
- Intramuscular (IM): 30 minutes - one hour after injection
- Oral: One hour after drug is taken (assumes a half-life of > two hours)