To assess steady-state digoxin concentration after initiation, when should the blood sample be collected?

• A. Immediately after the loading dose
• B. At least 6 hours after the oral dose
• C. 24 hours after the dose
• D. 2 hours after the dose

Answer: (B)

The key idea is to time the serum level so it reflects the amount of drug that has actually entered and equilibrated in the body after the loading dose, without waiting so long that elimination would alter the reading. Digoxin is absorbed and distributes over several hours, with peak levels typically reached within about 2–6 hours after an oral dose. Sampling around 6 hours after the dose allows the concentration to rise toward its therapeutic range produced by the loading dose, giving a meaningful assessment of whether that loading dose has achieved the desired level. If you sample immediately, absorption hasn’t yet occurred, yielding a falsely low reading; waiting 24 hours would be more reflective of trough timing before the next dose and may not accurately reflect the loading-dose effect.