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Difficulty: MediumNephrology

A 68-year-old man is evaluated in the intensive care unit for decreased urine output. He was admitted 4 days ago for septic shock secondary to severe community-acquired pneumonia and initially required norepinephrine for blood pressure support. He has been hemodynamically stable off vasopressors for the past 24 hours. His current blood pressure is 118/76 mm Hg and heart rate is 82/min. Laboratory studies show:

Serum sodium: 140 mEq/L
Serum blood urea nitrogen (BUN): 32 mg/dL
Serum creatinine: 2.0 mg/dL

Urine sodium: 63 mEq/L
Urine creatinine: 30 mg/dL

Urinalysis reveals muddy brown granular casts.

Calculate the patient's fractional excretion of sodium (FeNa). (Express the result as a percentage)

Answer: 3 %

Answer

3.0
The correct Fractional Excretion of Sodium (FeNa) is calculated using the formula FeNa=UNa×PCrPNa×UCr×100FeNa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100. Plugging in the patient's values yields 63×2.0140×30×100\frac{63 \times 2.0}{140 \times 30} \times 100, which simplifies to 1264200×100=3.0%\frac{126}{4200} \times 100 = 3.0\%. In the clinical context of a patient recovering from septic shock, a FeNa greater than 2% strongly points toward intrinsic acute kidney injury—specifically acute tubular necrosis (ATN). This diagnosis is further corroborated by the presence of muddy brown granular casts on urinalysis.

Step-by-Step Solution

1
Identify the standard formula for calculating FeNa.
FeNa=UNa×PCrPNa×UCr×100FeNa = \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 specific serum and urine chemistries into the formula.
FeNa=63×2.0140×30×100FeNa = \frac{63 \times 2.0}{140 \times 30} \times 100
Applying the clinical laboratory values provided in the vignette.
3
Calculate the products in the numerator and the denominator.
Numerator = 126126, Denominator = 42004200
To simplify the mathematical expression before final division.
4
Divide the numerator by the denominator and multiply by 100.
1264200×100=0.03×100=3.0\frac{126}{4200} \times 100 = 0.03 \times 100 = 3.0
To obtain the final percentage value for FeNa.

Key Concept

Fractional Excretion of Sodium (FeNa) Calculation
Estimated Time:1m 30s
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Topics

Nephrology

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