A 72-year-old woman is evaluated on hospital day 4 for worsening renal function. She was a… — USMLE Step 2 CK

A $72$ -year-old woman is evaluated on hospital day $4$ for worsening renal function. She was admitted for acute decompensated heart failure and has been receiving intravenous furosemide. Over the past $48$ hours, her weight has decreased by $4.5\text{ kg}$ , but her urine output has dropped to $300\text{ mL}$ over the last $12$ hours. Her blood pressure is $95/60\text{ mmHg}$ and heart rate is $102\text{/min}$ . Laboratory studies show:

Lab	Value
Serum sodium	$136\text{ mEq/L}$
Serum blood urea nitrogen (BUN)	$75\text{ mg/dL}$
Serum creatinine	$2.5\text{ mg/dL}$ (baseline $1.1\text{ mg/dL}$ )
Urine sodium	$110\text{ mEq/L}$
Urine urea nitrogen	$450\text{ mg/dL}$
Urine creatinine	$100\text{ mg/dL}$

What is this patient's fractional excretion of urea ( $FE_{Urea}$ )? (Express the answer as a percentage).

Answer: 15 %

Answer

The correct answer is

15\%

. This patient presents with acute kidney injury (AKI) in the setting of aggressive diuresis for heart failure. Distinguishing between prerenal azotemia (hypovolemia) and intrinsic renal disease (such as acute tubular necrosis) is critical for her ongoing management. While the fractional excretion of sodium (

FE_{Na}

) is typically less than

1\%

in prerenal AKI, loop diuretics directly inhibit sodium reabsorption, causing a falsely elevated

FE_{Na}

(in this patient's case, it would be calculated as roughly

2.02\%

). In patients on active diuretic therapy, the fractional excretion of urea (

FE_{Urea}

) is a much more accurate diagnostic marker because urea reabsorption is closely linked to proximal tubule water reabsorption and is significantly less affected by loop diuretics. An

FE_{Urea}

of less than

35\%

strongly suggests prerenal azotemia. The calculation is performed as follows:

FE_{Urea} = \frac{U_{Urea} \times P_{Cr}}{P_{Urea} \times U_{Cr}} \times 100 = \frac{450 \times 2.5}{75 \times 100} \times 100 = \frac{1125}{7500} \times 100 = 15\%

Step-by-Step Solution

Evaluate the clinical scenario and the limitation of standard AKI diagnostic indices.

Identify that the patient's active furosemide use falsely elevates sodium excretion, making FE_Na inaccurate (calculated here as 2.02%).

Loop diuretics inhibit sodium reabsorption in the thick ascending limb of the loop of Henle, invalidating FE_Na as a marker for volume status.

Select the appropriate diagnostic calculation for patients on diuretics.

Use the Fractional Excretion of Urea (FE_Urea).

Urea reabsorption occurs primarily in the proximal tubule and is less affected by loop diuretics. An FE_Urea < 35% reliably indicates a prerenal state.

Apply the FE_Urea formula.

FE_Urea = (Urine Urea_nitrogen * Serum Creatinine) / (Serum BUN * Urine Creatinine) * 100

This formula compares the clearance of urea to the clearance of creatinine to determine the fraction of filtered urea that is ultimately excreted.

Substitute the provided laboratory values into the formula.

(450 * 2.5) / (75 * 100) * 100 = 1125 / 7500 * 100

Accurate substitution of variables is required to solve the clearance ratio.

Perform the final calculation.

0.15 * 100 = 15%

To express the fractional excretion as a percentage.

Key Concept

In patients receiving active diuretic therapy, the fractional excretion of urea (FE_Urea) is a more reliable indicator of prerenal azotemia than the fractional excretion of sodium (FE_Na).

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