A 68-year-old man is admitted to the medical ward following a 4-day history of worsening d… — USMLE Step 2 CK

A 68-year-old man is admitted to the medical ward following a 4-day history of worsening diarrhea and generalized weakness. He states he has been unable to keep fluids down. His past medical history is significant for hypertension and osteoarthritis. His temperature is 37.0°C (98.6°F), blood pressure is 98/60 mm Hg, and pulse is 108/min. Jugular venous pressure is decreased, and skin turgor is poor. A Foley catheter is placed, yielding a small amount of concentrated urine.

Laboratory evaluation reveals the following:

Parameter	Value
Serum sodium	$140\text{ mEq/L}$
Serum potassium	$3.2\text{ mEq/L}$
Blood urea nitrogen	$45\text{ mg/dL}$
Serum creatinine	$1.5\text{ mg/dL}$
Urine sodium	$14\text{ mEq/L}$
Urine creatinine	$30\text{ mg/dL}$

What is this patient's fractional excretion of sodium?

Answer: 0.5 %

Answer

0.5

The fractional excretion of sodium (FeNa) measures the percentage of filtered sodium that is excreted in the urine. It is an essential calculation used to differentiate between prerenal and intrinsic etiologies of acute kidney injury. The formula is

FeNa = (U_{Na} / P_{Na}) \times (P_{Cr} / U_{Cr}) \times 100

. Substituting the provided laboratory values yields

(14 / 140) \times (1.5 / 30) \times 100 = 0.1 \times 0.05 \times 100 = 0.5\%

. A value less than

1\%

indicates avid sodium retention by the kidneys, which is characteristic of a prerenal state such as hypovolemia secondary to severe diarrhea.

Step-by-Step Solution

Identify the standard equation for fractional excretion of sodium.

FeNa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

This formula is used to differentiate the etiology of acute kidney injury.

Extract the specific patient values from the clinical vignette.

U_{Na} = 14\text{ mEq/L}

P_{Na} = 140\text{ mEq/L}

P_{Cr} = 1.5\text{ mg/dL}

U_{Cr} = 30\text{ mg/dL}

These are the four variables required to complete the calculation.

Substitute the parameters into the equation.

FeNa = \left(\frac{14}{140}\right) \times \left(\frac{1.5}{30}\right) \times 100

To set up the mathematical calculation.

Complete the mathematical operation.

FeNa = 0.1 \times 0.05 \times 100 = 0.5\%

To find the final diagnostic value, which supports a prerenal etiology.

Key Concept

Fractional Excretion of Sodium (FeNa) Calculation

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