What is the therapeutic range for digoxin in ng/mL?

• A. 1.0 to 3.0 ng/mL
• B. 0.1 to 0.5 ng/mL
• C. 0.5 to 2.0 ng/mL
• D. 2.0 to 4.0 ng/mL

Answer: (C)

Digoxin has a narrow therapeutic window, so serum levels are used to balance efficacy with safety. The range commonly cited for therapeutic effect is 0.5 to 2.0 ng/mL. Levels in this zone are high enough to provide the drug’s positive inotropic and rate-control effects without pushing toxicity too high.

In practice, many clinicians target the lower end (around 0.5–1.0 ng/mL) for heart failure to reduce the risk of side effects, while for atrial fibrillation the upper end (up to about 2.0 ng/mL) may be used if needed for rate control. Nevertheless, 0.5–2.0 ng/mL remains the general reference range.

Toxicity becomes more likely as levels rise above 2.0 ng/mL, and patients can experience GI symptoms, confusion, and, importantly, cardiac effects such as bradycardia, AV block, or other arrhythmias. Risk is higher with renal impairment, electrolyte disturbances (notably hypokalemia), and drug interactions (for example with amiodarone, quinidine, or verapamil) that raise digoxin levels. Because of this, monitoring trough levels after reaching steady state and adjusting dose carefully is essential.