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Difficulty: EasyNephrology

A 62-year-old man is evaluated in the hospital for decreased urine output over the past 24 hours. He was admitted three days ago for the treatment of community-acquired pneumonia with intravenous antibiotics. His blood pressure is 110/70 mm Hg and his pulse is 88/min. Laboratory studies show:

Serum sodium: 140 mEq/L140 \text{ mEq/L}
Serum creatinine: 4.0 mg/dL4.0 \text{ mg/dL}
Urine sodium: 70 mEq/L70 \text{ mEq/L}
Urine creatinine: 80 mg/dL80 \text{ mg/dL}

Based on these laboratory values, what is this patient's fractional excretion of sodium (in percent)?

Answer: 2.5 %

Answer

2.5
The correct calculation for the Fractional Excretion of Sodium (FENa) uses the formula (UNa×PCr)/(PNa×UCr)×100(U_{Na} \times P_{Cr}) / (P_{Na} \times U_{Cr}) \times 100. Plugging in the given values gives (70×4.0)/(140×80)×100=280/11200×100=2.5%(70 \times 4.0) / (140 \times 80) \times 100 = 280 / 11200 \times 100 = 2.5\%. A FENa greater than 2% in the setting of acute kidney injury is typically suggestive of acute tubular necrosis (ATN), which aligns with this patient's recent exposure to nephrotoxic intravenous antibiotics.

Step-by-Step Solution

1
Identify the mathematical formula for Fractional Excretion of Sodium (FENa).
FENa=UNa×PCrPNa×UCr×100 FENa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100
This formula relates the clearance of sodium to the clearance of creatinine to determine the percentage of filtered sodium that is excreted.
2
Substitute the patient's laboratory values into the FENa formula.
FENa=70×4.0140×80×100 FENa = \frac{70 \times 4.0}{140 \times 80} \times 100
Applying the specific serum and urine values provided in the clinical vignette.
3
Perform the calculation to find the final percentage.
FENa=28011200×100=0.025×100=2.5 FENa = \frac{280}{11200} \times 100 = 0.025 \times 100 = 2.5
Calculating the expression yields the final FENa percentage.

Key Concept

The Fractional Excretion of Sodium (FENa) is a calculated metric used to evaluate acute kidney injury, differentiating between prerenal (FENa < 1%) and intrinsic renal (FENa > 2%) etiologies.
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Topics

Nephrology

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