A hospital ward team is evaluating a 74-year-old female who was admitted three days ago fo… — USMLE Step 3

A hospital ward team is evaluating a 74-year-old female who was admitted three days ago for an acute exacerbation of chronic systolic heart failure. Since admission, she has received continuous intravenous bumetanide. Over the past 24 hours, her urine output has significantly decreased, and routine morning laboratory studies reveal an acute rise in her serum creatinine from a baseline of $1.1 \text{ mg/dL}$ .

The following laboratory values are obtained:

Parameter	Value
Serum sodium	$136 \text{ mEq/L}$
Serum potassium	$3.8 \text{ mEq/L}$
Serum chloride	$98 \text{ mEq/L}$
Blood urea nitrogen (BUN)	$72 \text{ mg/dL}$
Serum creatinine	$2.4 \text{ mg/dL}$
Urine sodium	$45 \text{ mEq/L}$
Urine creatinine	$60 \text{ mg/dL}$
Urine urea nitrogen	$450 \text{ mg/dL}$

Due to the confounding effect of ongoing loop diuretic therapy, the standard fractional excretion of sodium (FENa) may be misleading. The attending physician asks for the calculation of the most appropriate alternative diagnostic index to differentiate between prerenal azotemia and intrinsic acute tubular necrosis in this specific clinical context.

What is the value of this alternative index (expressed as a percentage)?

Answer: 25 %

Answer

The correct answer is 25. The patient has acute kidney injury in the setting of active loop diuretic therapy (bumetanide), which makes the fractional excretion of sodium (FENa) unreliable. The most appropriate alternative index is the fractional excretion of urea (FEUrea). Using the formula

\text{FE}_{\text{Urea}} = \frac{\text{Urine Urea} \times \text{Serum Cr}}{\text{Serum BUN} \times \text{Urine Cr}} \times 100

, the calculation is

\frac{450 \times 2.4}{72 \times 60} \times 100 = 25\%

Step-by-Step Solution

Determine the appropriate diagnostic index for acute kidney injury in a patient on active loop diuretic therapy.

The fractional excretion of urea (FEUrea) should be used instead of the fractional excretion of sodium (FENa).

Loop diuretics inhibit sodium reabsorption in the thick ascending limb, which falsely elevates urinary sodium excretion and FENa, rendering it unreliable for differentiating prerenal azotemia from acute tubular necrosis.

Recall the formula for the fractional excretion of urea.

\text{FE}_{\text{Urea}} = \frac{\text{Urine Urea} \times \text{Serum Cr}}{\text{Serum Urea} \times \text{Urine Cr}} \times 100

This formula calculates the clearance of urea relative to the clearance of creatinine.

Insert the provided laboratory values into the formula.

\text{FE}_{\text{Urea}} = \frac{450 \times 2.4}{72 \times 60} \times 100

Substituting urine urea nitrogen (

450 \text{ mg/dL}

), serum creatinine (

2.4 \text{ mg/dL}

), BUN/serum urea (

72 \text{ mg/dL}

), and urine creatinine (

60 \text{ mg/dL}

Perform the final arithmetic calculation.

\frac{1080}{4320} \times 100 = 0.25 \times 100 = 25\%

This yields the final percentage required by the question, correctly pointing to a prerenal etiology (value < 35%).

Key Concept

In patients receiving diuretic therapy, the fractional excretion of urea (FEUrea) is the most accurate diagnostic index to differentiate prerenal azotemia from acute tubular necrosis. A value less than 35% indicates a prerenal etiology.

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