During morning rounds in the medical intensive care unit, a physician evaluates a patient … — USMLE Step 2 CK

During morning rounds in the medical intensive care unit, a physician evaluates a patient admitted for severe hypovolemia secondary to intractable diarrhea.

The patient, a $55$ -year-old male, reports four days of unremitting watery stools. Intravenous fluid resuscitation is planned but has not yet been initiated. Vital signs show a blood pressure of $90/60\text{ mm Hg}$ and a heart rate of $110/\text{min}$ . Physical examination is notable for poor skin turgor, delayed capillary refill, and dry oral mucosa. Laboratory analysis of blood and urine samples obtained simultaneously reveals the following values:

Parameter	Value
Serum sodium	$135\text{ mEq/L}$
Serum creatinine	$2.7\text{ mg/dL}$
Blood urea nitrogen	$60\text{ mg/dL}$
Urine sodium	$27\text{ mEq/L}$
Urine creatinine	$135\text{ mg/dL}$

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

Answer: 0.4 %

Answer

The fractional excretion of sodium (FENa) is 0.4%.

The patient's clinical presentation (intractable diarrhea, hypotension, tachycardia, dry mucous membranes) is highly suggestive of prerenal acute kidney injury due to severe volume depletion. This is biochemically supported by calculating the fractional excretion of sodium (FENa), which measures the percentage of filtered sodium that is excreted in the urine. The formula is:

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

Plugging in the patient's values yields:

FeNa = \frac{27 \times 2.7}{135 \times 135} \times 100 = \frac{72.9}{18225} \times 100 = 0.4\%

A FENa of less than

1\%

indicates intense renal sodium avidity and reabsorption, which is the hallmark of a prerenal etiology as the kidneys attempt to restore intravascular volume. Note that while the absolute urine sodium concentration in this patient is greater than

20\text{ mEq/L}

(a classic but often unreliable indicator of prerenal status), the FENa calculation adjusts for free water reabsorption and accurately unmasks the true prerenal state.

Step-by-Step Solution

Identify the formula for the fractional excretion of sodium (FENa).

FeNa = (Urine Na * Serum Cr) / (Serum Na * Urine Cr) * 100

This formula calculates the percentage of filtered sodium that is excreted, which helps differentiate prerenal from intrinsic acute kidney injury.

Extract the necessary laboratory values from the clinical vignette.

Urine Na = 27 mEq/L, Serum Cr = 2.7 mg/dL, Serum Na = 135 mEq/L, Urine Cr = 135 mg/dL.

These specific values are required to populate the variables in the FENa equation. Note that Blood Urea Nitrogen (BUN) is not needed for this calculation.

Substitute the values into the formula and calculate the numerator and denominator.

Numerator = 27 * 2.7 = 72.9. Denominator = 135 * 135 = 18225.

Simplifying the fraction before final division ensures calculation accuracy.

Divide the numerator by the denominator and multiply by 100 to convert to a percentage.

(72.9 / 18225) * 100 = 0.004 * 100 = 0.4%.

This yields the final FENa percentage.

Key Concept

Fractional excretion of sodium (FENa) calculation in the assessment of acute kidney injury

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