A patient in the surgical intensive care unit develops oliguria on the second day following an elective partial colectomy. Over the past 8 hours, the urinary catheter has drained only 150 mL of highly concentrated urine. The patient has been receiving maintenance intravenous fluids at a conservative rate due to a known history of heart failure with preserved ejection fraction.
Vital signs show a temperature of 37.2°C (99.0°F), heart rate of 105/min, and blood pressure of 102/65 mmHg. Physical examination reveals dry oral mucosa and flat jugular veins.
Laboratory studies are obtained:
| Laboratory Parameter | Value |
|---|---|
| Serum sodium | 140 mEq/L |
| Serum blood urea nitrogen | 48 mg/dL |
| Serum creatinine | 2.0 mg/dL |
| Urine sodium | 28 mEq/L |
| Urine creatinine | 80 mg/dL |
Based on the provided laboratory values, calculate the patient's fractional excretion of sodium (FENa). (Enter your answer as a percentage.)
Answer: 0.5 %
Answer
0.5
The correct answer is derived mathematically using the Fractional Excretion of Sodium (FENa) formula. By multiplying the urine sodium by the serum creatinine (28 × 2.0 = 56) and dividing that product by the multiplication of the serum sodium and urine creatinine (140 × 80 = 11200), the resulting ratio is 0.005. Multiplying this decimal by 100 expresses the final FENa as 0.5%. Physiologically, a value below 1% indicates robust tubular sodium reabsorption, which strongly suggests a prerenal mechanism for the patient's oliguria, consistent with volume depletion.
Step-by-Step Solution
Key Concept
Calculation of the Fractional Excretion of Sodium (FENa) to evaluate and differentiate etiologies of acute kidney injury.