A 65-year-old man is admitted to the hospital for acute decompensated heart failure and is… — USMLE Step 2 CK

A 65-year-old man is admitted to the hospital for acute decompensated heart failure and is treated with aggressive intravenous furosemide. On hospital day 3, his blood pressure is 105/65 mm Hg and heart rate is 92/min. Over the past 24 hours, his urine output has decreased to 400 mL. His serum creatinine rises from a baseline of 1.1 mg/dL to 2.8 mg/dL.

Laboratory studies are obtained to evaluate his acute kidney injury:

Serum chemistry:
Sodium: 136 mEq/L
Potassium: 3.8 mEq/L
Chloride: 98 mEq/L
Bicarbonate: 28 mEq/L
Blood urea nitrogen (BUN): 84 mg/dL
Creatinine: 2.8 mg/dL

Urine chemistry:
Sodium: 70 mEq/L
Urea nitrogen: 420 mg/dL
Creatinine: 56 mg/dL

Because recent loop diuretic administration impairs tubular sodium reabsorption, the fractional excretion of sodium ( $FE_{Na}$ ) can be misleadingly elevated. Instead, the fractional excretion of urea ( $FE_{Urea}$ ) is preferred to differentiate between prerenal azotemia and intrinsic acute tubular necrosis in this clinical scenario.

Calculate the patient's $FE_{Urea}$ . Express your answer as a percentage.

Answer: 25 %

Answer

The correct calculation for

FE_{Urea}

[(420 \times 2.8) / (84 \times 56)] \times 100 = 25\%

. In the evaluation of acute kidney injury, establishing whether the etiology is prerenal (decreased perfusion) or intrinsic (tubular damage) is critical. While the fractional excretion of sodium (

FE_{Na}

) is standard, loop diuretics like furosemide block the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle, forcibly wasting sodium into the urine. This often results in an

FE_{Na} > 1\%

(in this patient, roughly 2.6%), which falsely points toward intrinsic damage. Because urea reabsorption occurs primarily in the proximal tubule and is largely unaffected by loop diuretics,

FE_{Urea}

remains a highly sensitive and specific metric. An

FE_{Urea}

less than 35% strongly confirms a prerenal state driven by severe heart failure and aggressive diuresis.

Step-by-Step Solution

Identify the correct formula for the Fractional Excretion of Urea (FEUrea).

FE_{Urea} = \left( \frac{Urine_{Urea} \times Serum_{Cr}}{Serum_{BUN} \times Urine_{Cr}} \right) \times 100

This formula standardizes urea clearance against creatinine clearance to assess proximal tubular handling of urea independent of sodium.

Extract the relevant variables from the provided laboratory data.

Urine Urea Nitrogen = 420 mg/dL; Serum Creatinine = 2.8 mg/dL; Serum BUN = 84 mg/dL; Urine Creatinine = 56 mg/dL.

Accurate identification is required to avoid incorrectly mixing serum and urine values during calculation.

Calculate the numerator by multiplying Urine Urea by Serum Creatinine.

420 \times 2.8 = 1176

This represents the relative amount of urea excreted compared to the serum creatinine level.

Calculate the denominator by multiplying Serum BUN by Urine Creatinine.

84 \times 56 = 4704

This represents the filtered load based on serum BUN normalized by the urine creatinine level.

Divide the numerator by the denominator and multiply by 100.

(1176 / 4704) \times 100 = 0.25 \times 100 = 25\%

This calculation yields the final percentage of filtered urea that is actually excreted by the kidneys.

Key Concept

Fractional Excretion of Urea (FEUrea) is a more reliable marker than FENa for differentiating prerenal azotemia from intrinsic acute tubular necrosis in patients actively receiving loop diuretics.

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