A 68-year-old man is brought to the emergency department due to profound lethargy and sign… — USMLE Step 2 CK

A 68-year-old man is brought to the emergency department due to profound lethargy and significantly decreased urine output for the past 2 days. The patient reports having 5 to 6 episodes of large-volume watery diarrhea daily for the last 4 days. His medical history is significant for hypertension, benign prostatic hyperplasia, and type 2 diabetes mellitus. His current medications include lisinopril, metformin, and tamsulosin.

On physical examination, his blood pressure is 92/60 mm Hg, heart rate is 112/min, and respiratory rate is 16/min. The patient's oral mucosa is extremely dry, skin turgor is poor, and jugular venous pressure is flat.

Laboratory results are as follows:

Test	Result
Serum
Sodium ( $P_{Na}$ )	135 mEq/L
Potassium	3.8 mEq/L
Chloride	105 mEq/L
Bicarbonate	20 mEq/L
Blood urea nitrogen	54 mg/dL
Creatinine ( $P_{Cr}$ )	1.8 mg/dL
Urine
Sodium ( $U_{Na}$ )	15 mEq/L
Creatinine ( $U_{Cr}$ )	100 mg/dL

Based on these findings, what is the calculated fractional excretion of sodium (FeNa) for this patient, expressed as a percentage?

Answer: 0.2 %

Answer

0.2

The correct calculation yields a fractional excretion of sodium (FeNa) of 0.2%. This patient presents with classic signs of severe hypovolemia (hypotension, tachycardia, flat neck veins, dry mucous membranes) secondary to profuse diarrhea. This volume depletion has led to prerenal acute kidney injury (AKI), as evidenced by an elevated BUN-to-creatinine ratio of 30:1 (normal is approximately 10:1 to 15:1). In states of systemic hypovolemia, the kidneys attempt to preserve intravascular volume by maximally reabsorbing filtered sodium and water under the influence of the renin-angiotensin-aldosterone system. This physiological response manifests as a highly concentrated urine with very low sodium content. A calculated FeNa of less than 1% effectively confirms appropriate intact tubular function responding to reduced renal perfusion, differentiating it from intrinsic tubular diseases like acute tubular necrosis, where the FeNa is typically greater than 2% due to impaired sodium reabsorption.

Step-by-Step Solution

Identify the values required for the fractional excretion of sodium equation from the provided lab results.

U_{Na}

= 15 mEq/L,

P_{Cr}

= 1.8 mg/dL,

P_{Na}

= 135 mEq/L, and

U_{Cr}

= 100 mg/dL.

These four variables are necessary to calculate the proportion of filtered sodium that is eventually excreted in the urine.

Set up the standard formula for fractional excretion of sodium (FeNa).

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

This formula represents the clearance of sodium divided by the glomerular filtration rate (estimated by creatinine clearance).

Substitute the extracted lab values into the FeNa formula and solve.

FeNa = \frac{15 \times 1.8}{135 \times 100} \times 100 = \frac{27}{13500} \times 100 = 0.2\%

Performing the arithmetic determines the precise percentage of filtered sodium that is being excreted.

Key Concept

The fractional excretion of sodium (FeNa) is calculated using the formula

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

. A FeNa < 1% indicates intense renal sodium conservation and strongly suggests a prerenal etiology for acute kidney injury.

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