A 64-year-old man is evaluated in the intensive care unit for a progressive decline in uri… — USMLE Step 3

A 64-year-old man is evaluated in the intensive care unit for a progressive decline in urine output. Three days ago, he underwent an emergent exploratory laparotomy for a perforated duodenal ulcer. His postoperative course was complicated by hypotension requiring vasopressor support for 24 hours, but he is currently hemodynamically stable. His urine output over the last 8 hours has been 120 mL.

Laboratory studies are obtained and reveal the following:

Serum chemistry
Sodium: $140$ mEq/L
Potassium: $4.8$ mEq/L
Chloride: $105$ mEq/L
Bicarbonate: $20$ mEq/L
BUN: $42$ mg/dL
Creatinine: $2.0$ mg/dL (baseline was $0.9$ mg/dL)

Urine studies
Sodium: $70$ mEq/L
Creatinine: $50$ mg/dL
Osmolality: $310$ mOsm/kg $H_2O$

Based on these laboratory results, what is the patient's fractional excretion of sodium (FENa)? Give your answer as a percentage.

Answer: 2 %

Answer

2.0

The correct calculation for the fractional excretion of sodium (FENa) uses the formula:

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

. Plugging in the patient's values gives

(70 \times 2.0) / (140 \times 50) \times 100 = 140 / 7000 \times 100 = 2.0\%

. A FENa of

2.0\%

or greater in this clinical context (recent prolonged hypotension leading to ischemic injury) strongly suggests acute tubular necrosis (ATN).

Step-by-Step Solution

Recall the formula for calculating the fractional excretion of sodium (FENa).

FENa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

FENa is a key clinical index utilized to determine if acute kidney injury is prerenal or secondary to intrinsic renal disease, such as acute tubular necrosis.

Identify the relevant serum and urine values from the laboratory results.

Urine sodium (

U_{Na}

) =

70

mEq/L; Serum creatinine (

P_{Cr}

) =

2.0

mg/dL; Serum sodium (

P_{Na}

) =

140

mEq/L; Urine creatinine (

U_{Cr}

) =

50

mg/dL.

These specific variables are necessary to accurately complete the calculation.

Substitute the extracted values into the formula.

FENa = \frac{70 \times 2.0}{140 \times 50} \times 100

Preparing the mathematical equation for simplification.

Calculate the products in the numerator and the denominator.

Numerator (

70 \times 2.0

) =

140

. Denominator (

140 \times 50

) =

7000

Simplifying the equation to a single fraction.

Divide the numerator by the denominator and multiply by

100

to obtain the final percentage.

140 / 7000 = 0.02

. Multiplying by

100

yields

2.0\%

Generates the final FENa percentage.

Key Concept

The fractional excretion of sodium (FENa) is calculated using the formula

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

. It is a vital laboratory calculation used to distinguish between prerenal azotemia (typically

<1\%

) and intrinsic renal disease like acute tubular necrosis (typically

>2\%

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