When is a blood sample for determination of the trough level of a drug appropriately drawn?

• A. During the absorption phase of the drug
• B. During the distribution phase of the drug
• C. Shortly before drug administration
• D. Two hours after drug administration

Answer: (C)

The key idea is to measure the lowest drug concentration in the dosing interval. A trough level should be drawn just before the next scheduled dose because that timing captures the minimum amount of drug remaining after elimination and distribution since the last dose. Sampling during the absorption or distribution phases would occur when drug levels are rising or still high, not representing the trough. Drawing two hours after administration is typically not the minimum and can depend on the drug’s kinetics, so it wouldn’t reliably reflect the trough. Measuring right before the next dose ensures the level is what the patient has at their lowest point, which is essential for adjusting the dose to stay within the therapeutic window.