When monitoring plasma digoxin concentration after a loading dose in a patient with heart failure, which is the most appropriate time to draw the blood sample?

• A. Immediately after the dose
• B. At least 6 hours after an oral dose has been administered
• C. 24 hours after the dose
• D. 2 hours after the oral dose

Answer: (B)

When checking digoxin after a loading dose, you want a blood sample that reflects the drug that has actually been absorbed and distributed, not the immediate absorption phase. Digoxin taken by mouth reaches its peak concentration over several hours, typically within about 2 to 6 hours. Waiting around 6 hours allows absorption to complete enough that the measured level represents the intended exposure from the loading dose, giving a meaningful indication of whether the loading dose has achieved therapeutic levels without being confounded by ongoing absorption.

If you draw immediately after the dose, you’d capture lower or unpredictable levels due to incomplete absorption. Drawing at 2 hours might still reflect the rising absorption phase and could mislead clinical decisions. Waiting 24 hours would capture a trough level before the next dose, which isn’t what you want when assessing the effect of a loading dose aimed at rapid stabilization. Therefore, timing the sample at least 6 hours after an oral dose best corresponds to the body’s exposure from the loading dose.