A 72-year-old man is admitted to the hospital with a 3-day history of severe diarrhea and … — USMLE Step 3

A 72-year-old man is admitted to the hospital with a 3-day history of severe diarrhea and decreased oral intake. His past medical history is significant for heart failure with preserved ejection fraction, for which he takes furosemide and lisinopril daily. On physical examination, his blood pressure is $90/60\text{ mm Hg}$ and heart rate is $112/\text{min}$ . Mucous membranes are dry, and capillary refill time is prolonged.

Initial laboratory studies are obtained:

Parameter	Serum Value	Urine Value
Sodium	$135\text{ mEq/L}$	$108\text{ mEq/L}$
Creatinine	$2.0\text{ mg/dL}$	$80\text{ mg/dL}$
Urea nitrogen (BUN)	$60\text{ mg/dL}$	$360\text{ mg/dL}$

The patient's baseline serum creatinine is $1.0\text{ mg/dL}$ . The physician suspects prerenal acute kidney injury secondary to volume depletion. Given the patient's active loop diuretic use, the fractional excretion of sodium (FENa) may be artifactually elevated and unreliable.

What is the calculated value (expressed as a percentage) of the alternative fractional excretion index that should be used to confirm the etiology of his kidney injury?

Answer: 15 %

Answer

The Fractional Excretion of Urea (FEUrea) is the most appropriate index here. Unlike sodium, urea reabsorption in the proximal tubule is largely uninhibited by loop and thiazide diuretics. The formula is

FEUrea = (U_{urea} \times P_{Cr}) / (P_{urea} \times U_{Cr}) \times 100

. Plugging in the values:

(360 \times 2.0) / (60 \times 80) \times 100 = 720 / 4800 \times 100 = 15\%

. An FEUrea

<35\%

is indicative of a prerenal state.

Step-by-Step Solution

Identify the correct diagnostic index

Fractional Excretion of Urea (FEUrea)

FENa is unreliable in patients on diuretics like furosemide because diuretic-induced natriuresis artificially elevates urine sodium. FEUrea is less affected by diuretic use.

Set up the mathematical formula for FEUrea

FEUrea = \frac{Urine Urea \times Serum Cr}{Serum BUN \times Urine Cr} \times 100

This formula calculates the percentage of filtered urea that is ultimately excreted in the urine.

Input the laboratory parameters and calculate

\frac{360 \times 2.0}{60 \times 80} \times 100 = 15\%

Plugging in the given values yields the final percentage required to confirm prerenal acute kidney injury.

Key Concept

In patients with acute kidney injury taking diuretics, Fractional Excretion of Urea (FEUrea) is a more reliable indicator of prerenal etiology than Fractional Excretion of Sodium (FENa). An FEUrea < 35% strongly suggests prerenal azotemia.

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