A nurse should plan to monitor which of these laboratory tests for a client who is receiving digoxin (Lanoxin)?

• A. Serum potassium
• B. Complete blood count
• C. Blood urea nitrogen (BUN)
• D. Serum glucose

Answer: (B)

Potassium level directly influences digoxin’s effect on the heart. Digoxin works by inhibiting the Na+/K+ ATPase pump in cardiac cells, which raises intracellular calcium and strengthens contractions. The amount of digoxin that can bind to that pump depends on the outside potassium level: when potassium is low, more binding sites are available, so the same digoxin dose can produce a stronger effect and a higher risk of toxicity. Maintaining serum potassium in the normal range helps prevent digoxin toxicity and ensures a safer, predictable response.

Therefore, monitoring serum potassium is the most relevant lab test for a client on digoxin. While kidney function and digoxin levels are also important in overall management, the electrolyte status most directly affects the safety and effectiveness of digoxin. A complete blood count assesses cellular components and isn’t directly tied to digoxin action, and other tests like BUN or serum glucose don’t reflect the immediate risk factors associated with digoxin therapy.