A 72-year-old man is evaluated in the hospital for decreased urine output. He was admitted… — USMLE Step 2 CK

A 72-year-old man is evaluated in the hospital for decreased urine output. He was admitted 3 days ago for an acute exacerbation of heart failure with reduced ejection fraction. His treatment regimen includes intravenous furosemide, lisinopril, carvedilol, and spironolactone. His weight has decreased by 4 kg (8.8 lb) since admission, and his peripheral edema has significantly improved. However, his urine output over the last 12 hours has been only 250 mL.

Vitals:
Blood pressure: 102/60 mm Hg
Heart rate: 94/min

Laboratory results from today:
Serum:
Sodium: 135 mEq/L
Potassium: 4.8 mEq/L
Chloride: 98 mEq/L
Bicarbonate: 28 mEq/L
Blood urea nitrogen: 80 mg/dL
Creatinine: 2.4 mg/dL

Urine:
Sodium: 75 mEq/L
Creatinine: 120 mg/dL
Urea nitrogen: 600 mg/dL

Based on the laboratory findings, what is this patient's calculated fractional excretion of urea (expressed as a percentage)?

Answer: 15 %

Answer

The correct calculation uses the formula

FeUrea = \frac{\text{Urine Urea Nitrogen} \times \text{Serum Creatinine}}{\text{Serum BUN} \times \text{Urine Creatinine}} \times 100

. Plugging in the provided values gives

\frac{600 \times 2.4}{80 \times 120} \times 100

, which simplifies to

\frac{1440}{9600} \times 100 = 15\%

. In a patient receiving loop diuretics (like furosemide), Fractional Excretion of Sodium (FeNa) is falsely elevated due to diuretic-induced natriuresis. Urea handling in the proximal tubule is unaffected by loop diuretics, making FeUrea a much more reliable indicator of prerenal azotemia. A value below

35\%

supports the diagnosis of prerenal acute kidney injury due to aggressive over-diuresis.

Step-by-Step Solution

Identify the clinical context and indication for calculating FeUrea.

The patient has AKI while on furosemide, making FeNa unreliable. FeUrea is indicated.

Loop diuretics inhibit Na+ reabsorption, falsely elevating FeNa even in prerenal states. Urea is reabsorbed proximally, independent of loop diuretics.

Recall the Fractional Excretion of Urea formula.

FeUrea = \frac{U_{Urea} \times P_{Cr}}{P_{BUN} \times U_{Cr}} \times 100

Fractional excretion compares the clearance of a substance (urea) to the clearance of creatinine.

Extract the correct values from the laboratory panel.

U_{Urea}

= 600 mg/dL,

P_{Cr}

= 2.4 mg/dL,

P_{BUN}

= 80 mg/dL,

U_{Cr}

= 120 mg/dL

Accurate variable selection is required; distractors like Urine Na and Serum Na must be ignored.

Execute the calculation.

\frac{600 \times 2.4}{80 \times 120} \times 100 = 15\%

Mathematical derivation of the fractional excretion percentage.

Key Concept

Fractional Excretion of Urea (FeUrea) is used to differentiate prerenal AKI from acute tubular necrosis in patients on diuretic therapy, where FeNa is often falsely elevated. A FeUrea < 35% indicates a prerenal etiology.

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