A 54-year-old woman is evaluated in the intensive care unit for worsening kidney function.… — USMLE Step 2 CK

A 54-year-old woman is evaluated in the intensive care unit for worsening kidney function. She was admitted four days ago for sepsis secondary to a complicated urinary tract infection and required aggressive fluid resuscitation and vasopressor support. Her urine output has steadily decreased over the past 24 hours.

Current laboratory results are as follows:
Serum sodium: 140 mEq/L
Serum creatinine: 2.1 mg/dL
Urine sodium: 60 mEq/L
Urine creatinine: 30 mg/dL

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

Answer: 3 %

Answer

The fractional excretion of sodium (FeNa) is 3%. The calculation is derived using the formula

FeNa = (U_{Na} \times P_{Cr}) / (P_{Na} \times U_{Cr}) \times 100

. Plugging in the values gives

(60 \times 2.1) / (140 \times 30) \times 100

, which simplifies to

126 / 4200 \times 100 = 3

. In this clinical context, a FeNa greater than 2% strongly points toward acute tubular necrosis (ATN) induced by previous ischemic shock, confirming intrinsic renal damage rather than a simple prerenal state.

Step-by-Step Solution

Identify the standard formula for calculating Fractional Excretion of Sodium (FeNa).

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 differentiate prerenal from intrinsic acute kidney injury.

Extract the specific laboratory values from the clinical vignette.

Urine sodium (

U_{Na}

) = 60, Plasma/Serum creatinine (

P_{Cr}

) = 2.1, Plasma/Serum sodium (

P_{Na}

) = 140, Urine creatinine (

U_{Cr}

) = 30.

These variables must be precisely assigned to avoid calculation errors.

Substitute the values into the formula and solve.

FeNa = \frac{60 \times 2.1}{140 \times 30} \times 100 = \frac{126}{4200} \times 100 = 0.03 \times 100 = 3

Solving the arithmetic yields the exact physiological percentage.

Key Concept

Calculation of Fractional Excretion of Sodium (FeNa)

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