A 78-year-old woman is evaluated in the hospital for worsening renal function. She was adm… — USMLE Step 2 CK

A $78$ -year-old woman is evaluated in the hospital for worsening renal function. She was admitted $4$ days ago for acute decompensated heart failure and has been receiving aggressive intravenous bumetanide therapy. Over the past $24$ hours, her urine output has decreased to $300\text{ mL}$ . On physical examination, her blood pressure is $105/65\text{ mmHg}$ and pulse is $92/\text{min}$ . Jugular venous pressure is undetectable, and her lungs are clear to auscultation. Laboratory studies are obtained:

Test	Result
Serum
Sodium	$140\text{ mEq/L}$
Blood urea nitrogen	$50\text{ mg/dL}$
Creatinine	$2.5\text{ mg/dL}$ (baseline $1.1\text{ mg/dL}$ )
Urine
Sodium	$112\text{ mEq/L}$
Urea nitrogen	$500\text{ mg/dL}$
Creatinine	$100\text{ mg/dL}$

Based on the laboratory values, calculate the fractional excretion of urea ( $FE_{Urea}$ ) as a percentage to help determine the etiology of her acute kidney injury.

Answer: 25 %

Answer

The fractional excretion of urea (FEUrea) is calculated using the formula:

FE_{Urea} = \frac{U_{Urea} \times P_{Cr}}{P_{Urea} \times U_{Cr}} \times 100

. Substituting the patient's laboratory values yields:

\frac{500 \times 2.5}{50 \times 100} \times 100 = \frac{1250}{5000} \times 100 = 25\%

. In patients receiving loop or thiazide diuretics, FEUrea is the preferred diagnostic metric over FeNa. Diuretics directly inhibit sodium reabsorption in the loop of Henle or distal convoluted tubule, artificially elevating urinary sodium and falsely increasing FeNa (in this patient, FeNa is calculated to be

2\%

). However, urea reabsorption occurs primarily in the proximal tubule and is largely unaffected by these diuretics. An

FE_{Urea} < 35\%

strongly suggests prerenal AKI, which perfectly aligns with this patient's clinical picture of over-diuresis (undetectable jugular venous pressure, cleared lungs, and relative hypotension).

Step-by-Step Solution

Assess the clinical scenario to choose the correct diagnostic calculation.

Determine that FEUrea should be used instead of FeNa.

The patient is actively receiving a loop diuretic (bumetanide), which artificially increases urinary sodium excretion and renders FeNa unreliable for diagnosing prerenal AKI.

Set up the Fractional Excretion of Urea (FEUrea) formula.

FE_{Urea} = \frac{U_{Urea} \times P_{Cr}}{P_{Urea} \times U_{Cr}} \times 100

This formula compares the clearance of urea to the clearance of creatinine to assess proximal tubular reabsorption.

Substitute the patient's laboratory values into the formula.

FE_{Urea} = \frac{500 \times 2.5}{50 \times 100} \times 100

Accurate placement of serum and urine values is necessary to perform the calculation.

Perform the arithmetic calculation.

FE_{Urea} = \frac{1250}{5000} \times 100 = 0.25 \times 100 = 25\%

To obtain the final percentage required by the question format.

Key Concept

In patients receiving loop diuretics, the Fractional Excretion of Urea (FEUrea) is a more reliable indicator than FeNa for distinguishing between prerenal AKI and intrinsic AKI. An FEUrea < 35% indicates a prerenal etiology.

Estimated Time:1m 30s

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