An elderly man is admitted due to a two-week history of worsening dyspnea, orthopnea, and … — USMLE Step 3

An elderly man is admitted due to a two-week history of worsening dyspnea, orthopnea, and increasing lower extremity edema. His past medical history is significant for ischemic cardiomyopathy with a left ventricular ejection fraction of $30\%$ . To manage his fluid overload at home, he recently doubled his daily dose of oral torsemide independently.

Upon admission, his blood pressure is $92/58\text{ mmHg}$ , pulse is $108/\text{min}$ , and respiratory rate is $22/\text{min}$ . Jugular venous pressure is visibly elevated. Lung auscultation reveals bilateral basilar crackles, and a third heart sound (S3) is present on cardiac exam.

Laboratory evaluation yields the following results:
Serum chemistry:
Sodium: $132\text{ mEq/L}$
Potassium: $3.1\text{ mEq/L}$
Chloride: $94\text{ mEq/L}$
Bicarbonate: $29\text{ mEq/L}$
Blood urea nitrogen (BUN): $75\text{ mg/dL}$
Creatinine: $2.5\text{ mg/dL}$ (baseline $1.2\text{ mg/dL}$ )

Spot urine chemistry:
Sodium: $66\text{ mEq/L}$
Urea nitrogen: $450\text{ mg/dL}$
Creatinine: $60\text{ mg/dL}$

To evaluate the etiology of the acute kidney injury in the setting of recent loop diuretic use, the medical team decides to calculate a specific fractional excretion index that is less confounded by his medication regimen than sodium excretion.

What is the calculated value of this preferred index, expressed as a percentage?

Answer: 25 %

Answer

The fractional excretion of urea (FEUrea) is the most accurate index to determine the etiology of acute kidney injury in patients actively receiving diuretic therapy. Diuretics increase distal sodium delivery and urinary sodium excretion, often rendering the FENa

> 1\%

even in severe prerenal states (like heart failure exacerbations). Urea reabsorption occurs primarily in the proximal tubule and is less affected by loop diuretics. The FEUrea is calculated as (Urine Urea Nitrogen

\times

Serum Creatinine) / (BUN

\times

Urine Creatinine)

\times

100. Plugging in the patient's values:

(450 \times 2.5) / (75 \times 60) \times 100 = 25\%

. A value

< 35\%

is strongly indicative of prerenal azotemia, consistent with this patient's cardiorenal syndrome.

Step-by-Step Solution

Identify the clinical problem and the limitation of standard diagnostic indices.

The patient has AKI and is on a loop diuretic, making the Fractional Excretion of Sodium (FENa) unreliable.

Loop diuretics block sodium reabsorption in the loop of Henle, causing natriuresis even when effective arterial blood volume is low, which can falsely elevate the FENa above

1\%

in prerenal states.

Select the appropriate alternative diagnostic index.

The Fractional Excretion of Urea (FEUrea) should be calculated.

Urea handling is linked to proximal tubular fluid reabsorption and is relatively unaffected by loop diuretics, making it a reliable marker for prerenal azotemia in this setting.

Recall the formula for FEUrea.

FE_{Urea} = \left( \frac{U_{Urea} \times P_{Cr}}{BUN \times U_{Cr}} \right) \times 100

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

Substitute the patient's laboratory values into the formula.

Numerator:

450 \times 2.5 = 1125

. Denominator:

75 \times 60 = 4500

. Ratio:

1125 / 4500 = 0.25

. As a percentage:

0.25 \times 100 = 25

Using consistent units (mg/dL) allows the units to cancel out, providing an accurate percentage. A value less than

35\%

confirms prerenal azotemia.

Key Concept

In patients with acute kidney injury who are receiving diuretic therapy, the fractional excretion of urea (FEUrea) is preferred over the fractional excretion of sodium (FENa) to distinguish prerenal azotemia from intrinsic acute tubular necrosis.

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