A 65-year-old man with normal renal function takes digoxin 0.25 mg daily and has a steady-state trough of 1.0 ng/mL. He develops progressive chronic kidney disease over months, halving his digoxin clearance. If the dose is left unchanged once the new steady state is reached, the expected trough concentration is approximately:

• A 0.5 ng/mL
• B 1.0 ng/mL
• C 2.0 ng/mL ✓
• D 4.0 ng/mL

Explanation

At steady state, the average concentration equals the dosing rate divided by clearance. Halving clearance while keeping the dose constant doubles the steady-state concentration to about 2.0 ng/mL, which also doubles toxicity risk for a narrow-index drug like digoxin. Note that the time to reach the new steady state lengthens because half-life inversely tracks clearance, but the magnitude of the final rise depends only on the clearance change. Option B ignores the altered clearance entirely.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

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Written and medically reviewed by the StethoPrep medical team.