A trough refers to the valley or bottom 'spike' of the saw-tooth 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 simply draw the sample right before the patient is supposed to take their next dose.
Assessing peak levels (the time of maximum concentration) is not as simple. For peak levels, the time of draw depends on the route of administration and the drug's absorption kinetics.
- For oral medications, one hour after the drug is taken is usually adequate (this assumes a half-life of >2 hours).
- For IV medications, sample 15–30 minutes after injection/infusion.
- For Intramuscular (IM) injections, sample 30 minutes to one hour after injection.
