Fractional Excretion of Sodium (FENa) helps differentiate between prerenal causes of acute kidney injury (where the kidneys appropriately reabsorb sodium) and intrinsic renal causes like acute tubular necrosis (where reabsorption is impaired).
Formula
FENa (%) = (Urine Na × Plasma Cr) / (Plasma Na × Urine Cr) × 100
Interpretation
- <1%: Suggests prerenal AKI (volume depletion, heart failure, hepatorenal syndrome)
- 1–2%: Indeterminate
- >2%: Suggests intrinsic renal injury (ATN, AIN, glomerular disease)
Important Limitations
FENa is unreliable in:
- Patients on diuretics (FEUrea is preferred — <35% suggests prerenal)
- Chronic kidney disease
- Contrast-induced nephropathy
- Pigmentary nephropathy (rhabdomyolysis, hemolysis)