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Difficulty: MediumNephrology

A 45-year-old man is evaluated in the intensive care unit for worsening kidney function. Three days ago, he was admitted with severe sepsis secondary to right lower lobe pneumonia and required vasopressor support for 48 hours. His blood pressure is now stable off vasopressors. Urine output over the last 12 hours has been 250 mL.

Laboratory studies are obtained:

Lab TestValue
Serum sodium140 mEq/L
Serum creatinine2.0 mg/dL
Blood urea nitrogen40 mg/dL
Urine sodium84 mEq/L
Urine creatinine60 mg/dL

What is the patient's fractional excretion of sodium (FeNaFe_{Na}) expressed as a percentage?

Answer: 2 %

Answer

2.0
The patient's fractional excretion of sodium (FeNaFe_{Na}) is calculated as 2.0%2.0\%. In the context of a patient with recent hypotension/shock, an FeNa≥2%Fe_{Na} \ge 2\% strongly points toward established Acute Tubular Necrosis (ATN) as the cause of acute kidney injury, reflecting the damaged tubules' inability to effectively reabsorb sodium.

Step-by-Step Solution

1
Recall the formula for Fractional Excretion of Sodium (FeNaFe_{Na}).
FeNa=UNa×PCrPNa×UCr×100Fe_{Na} = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100
This formula relates sodium clearance to creatinine clearance and is essential for distinguishing intrinsic renal disease from prerenal azotemia.
2
Extract the relevant values from the laboratory panel.
UNa=84U_{Na} = 84 mEq/L
PCr=2.0P_{Cr} = 2.0 mg/dL
PNa=140P_{Na} = 140 mEq/L
UCr=60U_{Cr} = 60 mg/dL
Accurate variable identification prevents common substitution errors, such as mistakenly using Blood Urea Nitrogen (BUN) instead of serum creatinine.
3
Substitute the values into the formula and calculate the numerator and denominator.
Numerator: 84×2.0=16884 \times 2.0 = 168
Denominator: 140×60=8400140 \times 60 = 8400
Breaking the calculation into intermediate steps reduces mathematical errors.
4
Divide the numerator by the denominator and multiply by 100.
1688400=0.02\frac{168}{8400} = 0.02
0.02×100=2.0%0.02 \times 100 = 2.0\%
Multiplying by 100 converts the raw fraction into the standard percentage format used in clinical practice.

Key Concept

Fractional Excretion of Sodium (FeNa) calculation and interpretation in Acute Kidney Injury

Alternative Method

You can also conceptualize FeNaFe_{Na} conceptually as the ratio of sodium clearance to creatinine clearance: FeNa=(CNa/CCr)×100Fe_{Na} = (C_{Na} / C_{Cr}) \times 100. This helps derive the equation intuitively without pure memorization.
Estimated Time:1m 0s
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Topics

Nephrology

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