Medical Diagnostics & Clinical Scoring

FENa (Fractional Excretion of Sodium)

Calculate FeNa to differentiate between pre-renal disease and acute tubular necrosis (ATN) as the cause of acute kidney injury.

FENa: 0.2%

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The Fractional Excretion of Sodium (FENa) is a classic nephrology calculation used to determine the etiology of Acute Kidney Injury (AKI).

Pre-Renal vs. Intrinsic AKI

When a patient's creatinine spikes, the primary question is: Are the kidneys themselves damaged, or are they just not getting enough blood flow?

  • Pre-renal AKI: The kidneys are perfectly healthy but are starving for blood (due to severe dehydration, hemorrhage, or heart failure). To compensate, the kidneys go into survival mode, aggressively reabsorbing almost all the sodium passing through them to hold onto water and raise blood pressure. Thus, very little sodium appears in the urine.
  • Intrinsic AKI (Acute Tubular Necrosis): The kidney tubules themselves have died or been damaged by toxins or prolonged ischemia. Even if the body wants to hold onto sodium, the broken tubules cannot physically reabsorb it. Thus, sodium spills out into the urine.

FENa (%) = [(Urine Na × Serum Creatinine) / (Serum Na × Urine Creatinine)] × 100

Where:
Urine Na=
Urine Sodium concentration.
Serum Na=
Serum Sodium concentration.

Interpreting FENa

  • < 1%: Pre-renal etiology. The kidneys are working hard to reabsorb sodium.
  • > 2%: Intrinsic etiology (ATN). The kidneys are damaged and inappropriately wasting sodium.
  • 1% to 2%: Indeterminate.

Frequently Asked Questions

The Fractional Excretion of Sodium (FENa) measures the percentage of sodium filtered by the kidneys that is actually excreted in the urine.

If the kidneys are healthy but the body is dehydrated (pre-renal), the kidneys will fiercely hold onto sodium to retain water, resulting in a FENa < 1%. If the kidneys themselves are damaged (intrinsic/ATN), the broken tubules leak sodium, resulting in a FENa > 2%.

No. Loop diuretics (like furosemide) artificially force the kidneys to excrete sodium. This will cause a falsely elevated FENa, making a pre-renal patient look like they have intrinsic damage. In these cases, the FEUrea should be used instead.

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