U01.21.031 Fractional Excretion of Sodium (FeNa⁺)

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Learning Objective:
Understand and calculate the Fractional Excretion of Sodium (FeNa⁺) — the percentage of filtered sodium that is excreted in the urine.

Formula:

FeNa^+ = \frac{V \times U_{Na}}{GFR \times P_{Na}} = \frac{P_{Cr} \times U_{Na}}{U_{Cr} \times P_{Na}}

Explanation:

  • FeNa^+ = Fractional excretion of sodium
  • V = Urine flow rate
  • U_{Na} = Urine sodium concentration
  • P_{Na} = Plasma sodium concentration
  • GFR = Glomerular filtration rate
  • P_{Cr} = Plasma creatinine
  • U_{Cr} = Urine creatinine

FeNa⁺ measures the percentage of filtered sodium excreted in the urine, indicating tubular function and sodium handling.

Concept Summary:

  • FeNa⁺ < 1% → Prerenal azotemia (intact tubular function, sodium reabsorption preserved)
  • FeNa⁺ > 2% → Intrinsic renal failure (tubular damage impairs sodium reabsorption)

Example:

Given:
U_{Na} = 40 \text{ mEq/L}
P_{Na} = 140 \text{ mEq/L}
U_{Cr} = 100 \text{ mg/dL}
P_{Cr} = 2 \text{ mg/dL}

FeNa^+ = \frac{2 \times 40}{100 \times 140} = 0.0057 = 0.57%

✅ Answer: FeNa⁺ = 0.57% → Prerenal pattern

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