A 68-year-old man develops oliguria and rising creatinine after CABG surgery. Urine output is 180 mL in 8 hours. Urine sodium is 62 mEq/L, urine creatinine 45 mg/dL, serum creatinine 3.2 mg/dL (baseline 1.0 mg/dL). Urine osmolality is 210 mOsm/kg. Based on these parameters, the fractional excretion of sodium (FENa) is approximately 3.2%, indicating:

• A Prerenal AKI (FENa <1%) — fluid resuscitation is indicated
• B Postrenal AKI — Foley catheter placement and imaging required
• C Cardiorenal syndrome type 1 — FENa <1% with elevated BNP
• D Intrinsic renal AKI (acute tubular necrosis) — FENa >2% with low urine osmolality ✓

Explanation

FENa = (UNa × PCr) / (PNa × UCr) × 100. With urine Na 62 mEq/L, serum Na ~140 mEq/L, urine Cr 45 mg/dL, serum Cr 3.2 mg/dL: FENa = (62 × 3.2) / (140 × 45) × 100 = 198.4/6300 × 100 ≈ 3.15%. FENa >2% with low urine osmolality (210 mOsm/kg, normal >500 in prerenal) indicates tubular injury with inability to concentrate urine — characteristic of acute tubular necrosis (ATN). Post-cardiac surgery ATN is common from ischemia-reperfusion injury. FENa <1% with high urine osmolality (>500) suggests prerenal. Note: FEUrea is more useful when diuretics are used (FEUrea <35% = prerenal). KDIGO defines AKI as Cr rise ≥0.3 mg/dL in 48h or ≥1.5× baseline in 7 days.

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

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