A 45-year-old male is evaluated in the surgical intensive care unit on postoperative day 3… — USMLE Step 2 CK

A 45-year-old male is evaluated in the surgical intensive care unit on postoperative day 3 following an extensive small bowel resection for mesenteric ischemia. Over the last 12 hours, the nursing staff reports that his urine output has decreased to $15 \text{ mL/hr}$ . He has been receiving maintenance intravenous fluids but has had large volumes of nasogastric tube output. His temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $90/55 \text{ mmHg}$ , and pulse is $118/\text{min}$ . Physical examination reveals flat jugular veins, delayed capillary refill, and dry mucous membranes.

Laboratory studies are obtained:

Parameter	Result
Serum sodium	$140 \text{ mEq/L}$
Serum blood urea nitrogen (BUN)	$42 \text{ mg/dL}$
Serum creatinine	$1.4 \text{ mg/dL}$
Urine sodium	$30 \text{ mEq/L}$
Urine creatinine	$60 \text{ mg/dL}$
Urine osmolality	$550 \text{ mOsm/kg}$

Based on the provided laboratory values, what is this patient's fractional excretion of sodium (FENa)? (Provide your answer as a numerical percentage, e.g., if the calculated value is 1.2%, enter 1.2)

Answer: 0.5 %

Answer

0.5

The fractional excretion of sodium (FENa) measures the percentage of filtered sodium that is eventually excreted in the urine. It is calculated using the formula:

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

. Plugging in the patient's specific lab values yields

\frac{30 \times 1.4}{140 \times 60} \times 100 = \frac{42}{8400} \times 100 = 0.5\%

. A FENa of less than

1\%

indicates appropriate and robust renal conservation of sodium, strongly suggesting a prerenal etiology for the acute kidney injury (AKI). This aligns with the patient's clinical presentation of significant hypovolemia due to poor intake and high nasogastric output.

Step-by-Step Solution

Recall the formula for Fractional Excretion of Sodium (FENa).

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

FENa evaluates acute kidney injury by comparing the fractional clearance of sodium to the fractional clearance of creatinine, which differentiates prerenal from intrinsic renal causes.

Extract the necessary variables from the patient's laboratory results.

Urine Sodium (

U_{Na}

) =

30 \text{ mEq/L}

, Serum Creatinine (

P_{Cr}

) =

1.4 \text{ mg/dL}

, Serum Sodium (

P_{Na}

) =

140 \text{ mEq/L}

, Urine Creatinine (

U_{Cr}

) =

60 \text{ mg/dL}

Accurate identification and isolation of the four required variables avoids calculation errors.

Input the variables into the FENa formula and perform the calculation.

\text{FENa} = \frac{30 \times 1.4}{140 \times 60} \times 100 = \frac{42}{8400} \times 100 = 0.005 \times 100 = 0.5\%

Solving the equation yields the physiological parameter required to determine the specific etiology of the patient's oliguria.

Key Concept

The fractional excretion of sodium (FENa) is an essential diagnostic calculation used to distinguish between prerenal acute kidney injury (FENa < 1%) and intrinsic acute tubular necrosis (FENa > 2%).

Estimated Time:1m 30s

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