A trough refers to the valley or bottom 'spike' of the sawtooth pattern we saw earlier. When a drug is taken orally the trough will be the low point of the serum concentration. It will be the time period immediately before the next dose. Thus, when we want to draw a trough level we draw just before the patient is supposed to take their next dose.
To assess peak levels (the time of maximum concentration) the time for drawing depends on the route of administration and the absorption kinetics of the drug.
- For oral medications, one hour after drug is taken is usually adequate (this assumes a half-life of > two hours).
- For IV medications, sample 15-30 minutes after injection/infusion.
- For Intramuscular (IM) injections, sample 30 minutes to one hour after injection.