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)?

Cevap: 0.5 %

Cevap

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.

Adım Adım Çözüm

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.

Anahtar Kavram

Calculation of Fractional Excretion of Sodium (FeNa)

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