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Difficulty: MediumNephrology

A 63-year-old man is brought to the emergency department by ambulance after vomiting a large amount of bright red blood. He has a history of alcohol use disorder and cirrhosis. On arrival, he is confused and lethargic. Temperature is 36.5°C (97.7°F), blood pressure is 85/50 mm Hg, and pulse is 124/min. Physical examination shows scleral icterus, spider angiomas on the chest, and a distended abdomen with a positive fluid wave. Two large-bore intravenous lines are placed, and fluid resuscitation is initiated.

Laboratory studies obtained prior to fluid administration show:

Serum:
Sodium: 140 mEq/L
Blood urea nitrogen: 68 mg/dL
Creatinine: 2.1 mg/dL

Urine:
Sodium: 12 mEq/L
Creatinine: 60 mg/dL

Calculate the patient's fractional excretion of sodium (in percent).

Answer: 0.3 %

Answer

0.3
The fractional excretion of sodium (FENa) is a useful calculation to distinguish prerenal acute kidney injury from intrinsic renal causes, such as acute tubular necrosis. A FENa of less than 1% typically indicates a prerenal state (e.g., hypovolemia or decreased effective circulating volume), where the kidneys appropriately and avidly reabsorb sodium to restore volume. In this patient with upper gastrointestinal bleeding and severe hypovolemia, the calculated FENa is 0.3%0.3\%, which strongly supports a prerenal etiology for his acute renal impairment.

Step-by-Step Solution

1
Identify the required variables for the FENa formula from the laboratory results.
UNaU_{Na} = 12 mEq/L, PCrP_{Cr} = 2.1 mg/dL, PNaP_{Na} = 140 mEq/L, UCrU_{Cr} = 60 mg/dL.
These specific serum and urine values are necessary to calculate the fractional excretion of sodium.
2
Set up the FENa formula.
FENa=UNa×PCrPNa×UCr×100FENa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100
This formula evaluates sodium clearance relative to creatinine clearance to estimate the percentage of filtered sodium that is excreted.
3
Perform the calculation.
FENa=12×2.1140×60×100=25.28400×100=0.003×100=0.3FENa = \frac{12 \times 2.1}{140 \times 60} \times 100 = \frac{25.2}{8400} \times 100 = 0.003 \times 100 = 0.3
Mathematically solving the equation yields the final percentage.

Key Concept

Fractional Excretion of Sodium (FENa) Calculation
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Topics

Nephrology

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