A 68-year-old man is evaluated in the hospital for decreased urine output. He was admitted… — USMLE Step 2 CK

A 68-year-old man is evaluated in the hospital for decreased urine output. He was admitted 3 days ago for severe gastroenteritis and has had poor oral intake. His blood pressure is $96/60 \text{ mm Hg}$ and heart rate is $112/\text{min}$ . Physical examination reveals dry oral mucosa and delayed capillary refill. Laboratory studies show:

Parameter	Value
Serum sodium	$140 \text{ mEq/L}$
Blood urea nitrogen	$60 \text{ mg/dL}$
Serum creatinine	$2.5 \text{ mg/dL}$
Urine sodium	$14 \text{ mEq/L}$
Urine creatinine	$50 \text{ mg/dL}$

Based on these laboratory findings, what is the patient's calculated fractional excretion of sodium (expressed as a percentage)?

Answer: 0.5 %

Answer

0.5

The calculated fractional excretion of sodium is 0.5%. A value less than 1% indicates appropriate renal sodium avidity in the setting of decreased renal perfusion, confirming prerenal acute kidney injury due to severe volume depletion from gastroenteritis.

Step-by-Step Solution

Identify the FeNa formula

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

This formula normalizes sodium excretion to glomerular filtration rate to evaluate acute kidney injury.

Extract laboratory values

U_{Na} = 14

P_{Cr} = 2.5

P_{Na} = 140

U_{Cr} = 50

These variables are required to calculate the fractional excretion.

Calculate the numerator

14 \times 2.5 = 35

Represents the product of urine sodium and plasma creatinine.

Calculate the denominator

140 \times 50 = 7000

Represents the product of plasma sodium and urine creatinine.

Compute final percentage

(\frac{35}{7000}) \times 100 = 0.5

Yields the fractional excretion expressed as a percentage.

Key Concept

Calculation of Fractional Excretion of Sodium (FeNa)

Estimated Time:1m 0s

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