In therapeutic drug monitoring, why are trough levels used?

• A. Because they reflect the maximum concentration during the dosing interval.
• B. Trough levels reflect the minimum concentration during the dosing interval and correlate with efficacy and toxicity risk; guide dose adjustments.
• C. They measure random fluctuations in concentration.
• D. They do not inform dosing decisions.

Answer: (B)

Trough levels are used in therapeutic drug monitoring because they represent the lowest drug concentration in a dosing interval, measured just before the next dose. This minimum level is crucial for ensuring the drug remains above the threshold needed for efficacy while avoiding concentrations that could cause toxicity. By monitoring the trough, clinicians can adjust the dose or dosing interval to keep drug exposure within the therapeutic range, especially for medications with narrow windows between effectiveness and harm. Trough measurements are reliable for guiding adjustments because they reflect how the drug is being cleared between doses and are less affected by the momentary peaks after dosing.

This is why the idea that troughs reflect the maximum concentration, measure random fluctuations, or fail to inform dosing decisions doesn’t fit: peaks show maximum concentrations, troughs are not about random fluctuations but the steady minimum just before the next dose, and trough data directly informs how to dose to maintain efficacy without toxicity.