A 68-year-old man is evaluated in the intensive care unit for oliguria. He was admitted 4 … — USMLE Step 2 CK

A 68-year-old man is evaluated in the intensive care unit for oliguria. He was admitted 4 days ago for severe acute pancreatitis complicated by shock, which required aggressive intravenous fluid resuscitation and transient vasopressor support. His blood pressure is now 112/70 mm Hg and his heart rate is 88/min on no vasoactive medications. Urine output over the past 24 hours has been 280 mL. Laboratory studies show the following:

Serum values:
- Sodium: $140$ mEq/L
- Potassium: $4.5$ mEq/L
- Chloride: $105$ mEq/L
- Bicarbonate: $21$ mEq/L
- Blood urea nitrogen: $48$ mg/dL
- Creatinine: $2.8$ mg/dL (baseline $0.9$ mg/dL)

Urine values:
- Sodium: $50$ mEq/L
- Creatinine: $50$ mg/dL

Urinalysis demonstrates the presence of muddy brown granular casts. Based on these findings, what is this patient's calculated fractional excretion of sodium (FENa) in percentage?

Answer: 2 %

Answer

The patient's fractional excretion of sodium (FENa) is 2%.

The correct calculation yields a fractional excretion of sodium of 2%. This patient has developed acute tubular necrosis (ATN), a form of intrinsic renal failure, as a complication of prolonged hypotension/shock (ischemic ATN). Intrinsic renal failure is characterized by structural damage to the renal tubules (evidenced by muddy brown granular casts). Because the damaged tubules cannot adequately reabsorb sodium, the FENa is typically > 2% in ATN. FENa is determined by the formula:

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

. Substituting the values from the vignette gives

\frac{50 \times 2.8}{140 \times 50} \times 100 = 2\%

Step-by-Step Solution

Extract the four relevant laboratory values required for the FENa calculation.

U_{Na} = 50

P_{Cr} = 2.8

P_{Na} = 140

, and

U_{Cr} = 50

The fractional excretion of sodium relies exclusively on urine and serum concentrations of sodium and creatinine.

Apply the standard formula for FENa.

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

This formula effectively calculates the clearance of sodium divided by the clearance of creatinine, estimating the proportion of filtered sodium that is excreted.

Calculate the numerator.

50 \times 2.8 = 140

Multiplies urine sodium by serum creatinine.

Calculate the denominator.

140 \times 50 = 7000

Multiplies serum sodium by urine creatinine.

Divide the numerator by the denominator and multiply by 100 to get the percentage.

\frac{140}{7000} \times 100 = 0.02 \times 100 = 2\%

Yields the final percentage indicating the diagnostic category of the acute kidney injury.

Key Concept

Calculation and interpretation of Fractional Excretion of Sodium (FENa) to distinguish intrinsic acute tubular necrosis (ATN) from prerenal azotemia.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic