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Difficulty: HardNephrology

A 68-year-old man is admitted to the hospital with acute decompensated heart failure. He is started on an intravenous bumetanide infusion. On hospital day 3, his urine output decreases to 300 mL over the last 24 hours. Physical examination shows jugular venous distention, bibasilar crackles, and bilateral lower extremity pitting edema.

Laboratory studies are obtained:
Serum sodium: 140 mEq/L140 \text{ mEq/L}
Serum potassium: 3.5 mEq/L3.5 \text{ mEq/L}
Serum chloride: 98 mEq/L98 \text{ mEq/L}
Serum bicarbonate: 26 mEq/L26 \text{ mEq/L}
Serum blood urea nitrogen (BUN): 60 mg/dL60 \text{ mg/dL}
Serum creatinine: 2.0 mg/dL2.0 \text{ mg/dL}

A spot urine sample is analyzed:
Urine sodium: 105 mEq/L105 \text{ mEq/L}
Urine blood urea nitrogen (BUN): 600 mg/dL600 \text{ mg/dL}
Urine creatinine: 100 mg/dL100 \text{ mg/dL}

Based on these findings, what is the patient's fractional excretion of urea nitrogen (expressed as a percentage)?

Answer: 20 %

Answer

20
The correct calculation for the fractional excretion of urea nitrogen is 20%20\%. To determine the etiology of acute kidney injury (AKI) in a patient receiving diuretic therapy, the fractional excretion of urea (FeUreaFeUrea) should be used rather than the fractional excretion of sodium (FeNaFeNa). Diuretics impair sodium reabsorption in the tubules, which can falsely elevate the urine sodium concentration and the FeNaFeNa, masking a prerenal state. Urea reabsorption, primarily occurring in the proximal tubule, is less affected by diuretic use. The formula is FeUrea=UrineBUN×SerumCrSerumBUN×UrineCr×100FeUrea = \frac{Urine_{BUN} \times Serum_{Cr}}{Serum_{BUN} \times Urine_{Cr}} \times 100. Plugging in the provided values gives 600×2.060×100×100=20%\frac{600 \times 2.0}{60 \times 100} \times 100 = 20\%. A FeUreaFeUrea of less than 35%35\% is indicative of prerenal AKI, which is consistent with this patient's presentation of low forward cardiac output in the setting of acute decompensated heart failure.

Step-by-Step Solution

1
Identify the indication for calculating FeUreaFeUrea instead of FeNaFeNa.
The patient is on bumetanide (a loop diuretic).
Diuretics increase urinary sodium excretion, making FeNaFeNa unreliable (falsely elevated) for diagnosing prerenal AKI. Urea reabsorption is less affected by diuretics.
2
Select the correct formula for FeUreaFeUrea.
FeUrea=UrineBUN×SerumCrSerumBUN×UrineCr×100FeUrea = \frac{Urine_{BUN} \times Serum_{Cr}}{Serum_{BUN} \times Urine_{Cr}} \times 100
This formula compares the clearance of urea to the clearance of creatinine.
3
Plug the laboratory values into the formula.
FeUrea=600×2.060×100×100FeUrea = \frac{600 \times 2.0}{60 \times 100} \times 100
Extract values: UrineBUN=600Urine_{BUN} = 600, SerumCr=2.0Serum_{Cr} = 2.0, SerumBUN=60Serum_{BUN} = 60, UrineCr=100Urine_{Cr} = 100.
4
Perform the calculation.
12006000×100=0.2×100=20\frac{1200}{6000} \times 100 = 0.2 \times 100 = 20
Basic arithmetic yields the final percentage.

Key Concept

In patients receiving diuretics who develop acute kidney injury, the fractional excretion of urea (FeUreaFeUrea) is a more accurate marker for prerenal etiology than the fractional excretion of sodium (FeNaFeNa). A FeUrea<35%FeUrea < 35\% indicates prerenal AKI.
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Topics

Nephrology

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