A 68-year-old man presents to the clinic with progressive fatigue, lightheadedness, and re… — USMLE Step 3

A $68$ -year-old man presents to the clinic with progressive fatigue, lightheadedness, and reduced urine output over the past three days. His medical history is significant for ischemic cardiomyopathy and type 2 diabetes mellitus. His daily medications include aspirin, lisinopril, empagliflozin, and furosemide $40$ mg twice daily.

Vital signs:
Blood pressure: $92/60$ mmHg
Heart rate: $105$ /min
Respiratory rate: $16$ /min
Temperature: $36.8^{\circ}$ C ( $98.2^{\circ}$ F)

Physical examination reveals dry mucous membranes, decreased skin turgor, and flat neck veins.

Laboratory results:

Test	Value
Serum sodium	$136$ mEq/L
Serum potassium	$3.4$ mEq/L
Serum chloride	$98$ mEq/L
Serum bicarbonate	$28$ mEq/L
Blood urea nitrogen	$42$ mg/dL
Serum creatinine	$2.1$ mg/dL
Urine sodium	$45$ mEq/L
Urine creatinine	$50$ mg/dL
Urine urea nitrogen	$250$ mg/dL

Because the patient is receiving active diuretic therapy, the fractional excretion of sodium (FENa) is considered unreliable for determining the etiology of his acute kidney injury.

What is this patient's fractional excretion of urea (expressed as a percentage)?

Answer: 25 %

Answer

The fractional excretion of urea (FEUrea) is 25%.

The fractional excretion of urea (FEUrea) is calculated using the formula:

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

. Plugging in the patient's values gives:

\frac{250 \times 2.1}{42 \times 50} \times 100 = \frac{525}{2100} \times 100 = 25\%

. In a patient taking furosemide, renal sodium reabsorption is pharmacologically inhibited, rendering FENa unreliable. Urea handling, however, is largely unaffected by loop diuretics, making an FEUrea of less than

35\%

a reliable indicator of prerenal azotemia.

Step-by-Step Solution

Select the correct clinical calculation.

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

The patient is on loop diuretic therapy (furosemide), which artificially increases urine sodium and invalidates the FENa calculation. FEUrea is the preferred alternative index to evaluate for prerenal azotemia in this setting.

Extract the corresponding laboratory values from the clinical vignette.

Urine urea nitrogen (

U_{Urea}

) =

250

mg/dL, Serum creatinine (

P_{Cr}

) =

2.1

mg/dL, Serum BUN (

P_{BUN}

) =

42

mg/dL, Urine creatinine (

U_{Cr}

) =

50

mg/dL.

These specific values are the necessary variables for the FEUrea equation.

Perform the mathematical calculation.

FEUrea = \frac{250 \times 2.1}{42 \times 50} \times 100 = \frac{525}{2100} \times 100 = 25\%

Computing this ratio determines the percentage of filtered urea that is actually excreted. A value

< 35\%

confirms a prerenal etiology for the acute kidney injury.

Key Concept

In patients with acute kidney injury taking diuretics, the fractional excretion of urea (FEUrea) is a more accurate marker for differentiating prerenal azotemia from intrinsic renal disease than the fractional excretion of sodium (FENa). An FEUrea less than 35% indicates prerenal azotemia.

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