What is a trough concentration, and why is it important?

• A. The lowest drug concentration just before the next dose; used in TDM to keep within therapeutic range.
• B. The highest drug concentration after dosing; used to assess maximum exposure.
• C. The average concentration over a dosing interval.
• D. The concentration at which the drug is fully metabolized.

Answer: (A)

Trough concentration is the lowest drug level in a dosing interval, measured just before the next dose is given. This timing captures how much drug remains after the body has had time to clear it since the last dose. It’s important for therapeutic drug monitoring because it helps ensure levels stay within the therapeutic range: high enough to be effective but low enough to avoid toxicity. By watching troughs, clinicians can adjust dose or dosing frequency, especially for drugs with a narrow therapeutic window or variable pharmacokinetics. This concept is different from peak concentration, which is the highest level after dosing, and from the average exposure over a dosing interval.