The plasma drug concentration assay for this drug should be taken at least 6 hours after a dose has been administered. Which drug is this?

• A. Warfarin
• B. Theophylline
• C. Lidocaine
• D. Digoxin

Answer: (D)

The timing of a plasma concentration check is chosen to reflect the trough level—that point in the dosing cycle when the drug concentration is most informative for guiding therapy and minimizing toxicity. For digoxin, this matters because the drug has a long half-life and a very narrow therapeutic window. After dosing, digoxin reaches a relatively early peak, but with ongoing dosing it accumulates, and the value that best represents ongoing exposure is the trough—measured just before the next dose. Checking the level at least six hours after a dose helps ensure you’re past the absorption peak and capturing a trough that can be reliably compared to the target range (roughly 0.8–2.0 ng/mL in adults). This trough at steady state is what clinicians use to adjust dosing and avoid toxicity.