Examkin

Question

Difficulty: EasyNephrology

A 54-year-old man is hospitalized for the management of severe pneumonia complicated by sepsis. On hospital day 3, his urine output drops to 200200 mL over a 12-hour period. To evaluate the etiology of his oliguric acute kidney injury, the following laboratory studies are obtained:

Serum sodium: 140140 mEq/L
Serum creatinine: 2.02.0 mg/dL
Urine sodium: 5656 mEq/L
Urine creatinine: 4040 mg/dL

What is this patient's fractional excretion of sodium (FENa)? Give your answer as a percentage.

Answer: 2 %

Answer

2.0
The correct value is 2.0%. The Fractional Excretion of Sodium (FENaFE_{Na}) measures the percentage of filtered sodium that is ultimately excreted in the urine. It is calculated using the formula: FENa=UNa×SCrSNa×UCr×100FE_{Na} = \frac{U_{Na} \times S_{Cr}}{S_{Na} \times U_{Cr}} \times 100. By substituting the patient's specific laboratory values into this formula, we get 56×2.0140×40×100=1125600×100=2.0%\frac{56 \times 2.0}{140 \times 40} \times 100 = \frac{112}{5600} \times 100 = 2.0\%. In the context of oliguric acute kidney injury, an FENaFE_{Na} of ≥2%\geq 2\% points toward an intrinsic renal etiology, which aligns perfectly with acute tubular necrosis secondary to this patient's sepsis.

Step-by-Step Solution

1
Identify the proper formula for calculating the Fractional Excretion of Sodium.
FENa=UNa×SCrSNa×UCr×100FE_{Na} = \frac{U_{Na} \times S_{Cr}}{S_{Na} \times U_{Cr}} \times 100
This formula helps differentiate between prerenal azotemia and intrinsic renal failure (like acute tubular necrosis).
2
Substitute the patient's serum and urine laboratory values into the equation.
FENa=56×2.0140×40×100FE_{Na} = \frac{56 \times 2.0}{140 \times 40} \times 100
Applying the specific clinical data provided in the stem.
3
Perform the mathematical calculation.
1125600×100=0.02×100=2.0\frac{112}{5600} \times 100 = 0.02 \times 100 = 2.0
To obtain the final percentage value.

Key Concept

Calculation of Fractional Excretion of Sodium (FENa) to evaluate acute kidney injury
Rate this question

Topics

Nephrology

More questions from this topic