Two days after major abdominal surgery, a 60-year-old man passes 20 mL of urine per hour. Urine sodium is 15 mEq/L, urine creatinine 120 mg/dL, plasma sodium 140 mEq/L, plasma creatinine 1.5 mg/dL. Calculate the fractional excretion of sodium and state its implication.

• A FENa approximately 3.6 percent, consistent with acute tubular necrosis
• B FENa approximately 0.13 percent, consistent with prerenal azotemia from volume depletion ✓
• C FENa approximately 1.3 percent, indeterminate and requiring renal biopsy
• D FENa approximately 0.9 percent, consistent with contrast-induced nephropathy

Explanation

FENa = (UNa x PCr) / (PNa x UCr) x 100 = (15 x 1.5) / (140 x 120) x 100 = 22.5 / 16800 x 100, which is about 0.13 percent. A FENa below 1 percent indicates avid tubular sodium reabsorption, the signature of prerenal azotemia from hypoperfusion, common after surgery with third-space losses. Acute tubular necrosis typically gives a FENa above 2 percent because damaged tubules cannot reclaim sodium.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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