Final answer:
A trough sample for routine therapeutic drug monitoring is usually obtained just before the next scheduled dose to measure the drug concentration at its lowest level. Sampling at random times during the day may not provide accurate information.
Step-by-step explanation:
In routine therapeutic drug monitoring, a trough sample is usually obtained just before the next scheduled dose.
This is done to measure the drug concentration at its lowest level, right before the next dose is given. By measuring the trough concentration, healthcare professionals can determine if the drug is being cleared from the body at the desired rate and whether the dosage needs to be adjusted.
Sampling at random times during the day may not provide accurate information about the drug's concentration.