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Difficulty: MediumNephrology

A 55-year-old man presents to the emergency department with a 4-day history of profuse watery diarrhea. He reports orthostatic dizziness, generalized weakness, and a significantly reduced urine output over the past 24 hours. He has no significant past medical history and takes no medications. Vital signs show a temperature of 37.1∘C37.1^\circ\text{C} (98.8∘F98.8^\circ\text{F}), blood pressure of 88/55 mm Hg88/55\text{ mm Hg}, and a heart rate of 122/min122/\text{min}. Physical examination reveals dry mucous membranes, decreased skin turgor, and flat jugular veins.

Laboratory studies are as follows:

Serum
Sodium: 140 mEq/L140\text{ mEq/L}
Potassium: 3.2 mEq/L3.2\text{ mEq/L}
Chloride: 110 mEq/L110\text{ mEq/L}
Bicarbonate: 18 mEq/L18\text{ mEq/L}
Blood urea nitrogen (BUN): 60 mg/dL60\text{ mg/dL}
Creatinine: 2.5 mg/dL2.5\text{ mg/dL}

Urine
Sodium: 14 mEq/L14\text{ mEq/L}
Creatinine: 50 mg/dL50\text{ mg/dL}

What is this patient's fractional excretion of sodium (FeNaFe_{Na}) in percentage?

Answer: 0.5 %

Answer

0.5
The fractional excretion of sodium (FeNaFe_{Na}) measures the percentage of filtered sodium that is eventually excreted in the urine. It is widely used to distinguish between prerenal acute kidney injury (AKI) and acute tubular necrosis (ATN). It is calculated using the formula: FeNa=UNa×PCrPNa×UCr×100Fe_{Na} = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100. Plugging in the patient's laboratory values yields 14×2.5140×50×100=0.5%\frac{14 \times 2.5}{140 \times 50} \times 100 = 0.5\%. In the setting of oliguria, an FeNa<1%Fe_{Na} < 1\% typically indicates prerenal azotemia, representing an intact renal tubular mechanism trying to conserve sodium and water in response to hypovolemia (consistent with this patient's severe dehydration from diarrhea).

Step-by-Step Solution

1
Identify the values required for the FeNa calculation.
UNa=14U_{Na} = 14, PCr=2.5P_{Cr} = 2.5, PNa=140P_{Na} = 140, UCr=50U_{Cr} = 50
These specific variables are needed to use the FeNaFe_{Na} equation, which helps differentiate prerenal azotemia from intrinsic renal disease.
2
Apply the FeNa formula.
FeNa=14×2.5140×50×100Fe_{Na} = \frac{14 \times 2.5}{140 \times 50} \times 100
The standard formula requires multiplying the ratio of urine-to-plasma sodium by the ratio of plasma-to-urine creatinine.
3
Calculate the final percentage.
0.5%0.5\%
This calculation yields the final percentage, which is less than 1%1\% and highly indicative of a prerenal etiology.

Key Concept

Fractional Excretion of Sodium (FeNa) Calculation
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Topics

Nephrology

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