A 74-year-old man with stable angina and COPD has worsening edema and signs of heart failure. Loop diuretic therapy is started. Which laboratory parameter must be monitored regularly after starting treatment?

• A. Serum calcium
• B. Serum magnesium
• C. Serum potassium
• D. Hematocrit

Answer: (C)

Loop diuretics help unload excess fluid by blocking the Na-K-2Cl transporter in the thick ascending limb, which not only increases sodium and water excretion but also pushes potassium (and magnesium) loss into the urine. Because potassium is essential for normal cardiac electrical activity, dropping serum potassium can lead to dangerous arrhythmias, especially in an older patient with coronary disease. That makes regular monitoring of serum potassium the most important lab check after starting a loop diuretic. Magnesium can also be affected, so clinicians may monitor it too, but potassium is the primary parameter to watch to prevent life-threatening rhythm disturbances. Hematocrit and calcium levels can be influenced by diuresis or other factors, but they do not carry the same immediate risk from loop diuretics as potassium does in this scenario.