A 75-year-old woman is evaluated in the hospital for acute kidney injury. She was admitted… — USMLE Step 3

A 75-year-old woman is evaluated in the hospital for acute kidney injury. She was admitted $5$ days ago for community-acquired pneumonia and has been treated with intravenous vancomycin and piperacillin-tazobactam. Her blood pressure is $122/76$ mm Hg, and heart rate is $82$ /min. She appears euvolemic on physical examination. Today, her serum creatinine is noted to be significantly elevated compared to her baseline of $0.9$ mg/dL.

Laboratory studies today show:
Serum sodium: $140$ mEq/L
Blood urea nitrogen: $56$ mg/dL
Serum creatinine: $4.0$ mg/dL
Urine sodium: $63$ mEq/L
Urine creatinine: $60$ mg/dL

Urinalysis reveals muddy brown granular casts.

Based on the provided laboratory data, what is the patient's fractional excretion of sodium (FENa) as a percentage?

Answer: 3 %

Answer

3.0

The correct calculation yields 3.0%. FENa is a diagnostic calculation used to differentiate causes of acute kidney injury. It is calculated using the formula

FENa = \frac{UNa \times SCr}{SNa \times UCr} \times 100

. For this patient, who likely has nephrotoxic acute tubular necrosis induced by vancomycin and piperacillin-tazobactam (supported by muddy brown casts), substituting the provided values results in

\frac{63 \times 4.0}{140 \times 60} \times 100 = 3.0\%

Step-by-Step Solution

Recall the diagnostic formula for calculating the fractional excretion of sodium (FENa).

FENa = \frac{UNa \times SCr}{SNa \times UCr} \times 100

This calculation is essential to differentiate the etiology of acute kidney injury, specifically distinguishing prerenal azotemia from intrinsic renal disease such as acute tubular necrosis.

Extract the necessary laboratory parameters from the clinical vignette.

Urine sodium (

UNa

) =

63

mEq/L, Serum creatinine (

SCr

) =

4.0

mg/dL, Serum sodium (

SNa

) =

140

mEq/L, and Urine creatinine (

UCr

) =

60

mg/dL.

Accurate identification of values is required for the calculation; the blood urea nitrogen (BUN) is provided as a distractor and is not part of the FENa equation.

Substitute the values into the formula and perform the calculation.

FENa = \frac{63 \times 4.0}{140 \times 60} \times 100 = \frac{252}{8400} \times 100 = 0.03 \times 100 = 3.0\%

Arithmetic completion determines the final index, which is consistent with the clinical picture of acute tubular necrosis (FENa > 2%).

Key Concept

The fractional excretion of sodium (FENa) measures the percentage of filtered sodium excreted in the urine. It is calculated as

FENa = \frac{\text{Urine Na} \times \text{Serum Cr}}{\text{Serum Na} \times \text{Urine Cr}} \times 100

. A value > 2% typically indicates intrinsic renal injury, such as acute tubular necrosis.

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