A nurse is caring for a client who has heart failure and a new prescription for furosemide. Which of the following laboratory values should the nurse review before administering furosemide?

Choice A rationale Phosphate levels are generally not the primary concern when administering loop diuretics like furosemide. While diuretics can affect various electrolytes, the most significant life-threatening imbalances caused by furosemide involve potassium and sodium. Normal serum phosphate ranges from 3.0 to 4.5 mg/dL. While it is important to monitor overall metabolic status in heart failure patients, phosphate does not have the same immediate impact on cardiac rhythm and muscle function as potassium does during diuretic therapy. Choice B rationale Carbon dioxide levels in a metabolic panel reflect the bicarbonate concentration and the acid-base balance of the blood. While loop diuretics can cause metabolic alkalosis, which would increase these levels, it is not the most critical parameter to check before a single dose. Normal venous carbon dioxide levels range from 23 to 30 mEq/L. The priority is to assess for electrolyte deficiencies that could cause immediate cardiac complications, making potassium a more vital laboratory value to review. Choice C rationale Bicarbonate levels are an indicator of the body's acid-base status. Furosemide can lead to contraction alkalosis, causing an increase in bicarbonate levels over time. Normal bicarbonate levels range from 22 to 28 mEq/L. While monitoring for alkalosis is part of long-term care for clients on diuretics, it is not the priority assessment before drug administration. The nurse must prioritize electrolytes that directly influence heart rate and rhythm in the context of heart failure and loop diuretic use. Choice D rationale Furosemide is a loop diuretic that inhibits the reabsorption of sodium, chloride, and water in the ascending limb of the loop of Henle, leading to increased excretion of potassium. Normal serum potassium levels are 3.5 to 5.0 mEq/L. Administering furosemide to a client with hypokalemia can lead to life-threatening cardiac arrhythmias. Because heart failure patients often take other medications like digoxin, low potassium significantly increases the risk of toxicity and must be verified before administration.