Calculate the Fractional Excretion of Sodium (FeNa) to help differentiate prerenal azotemia from acute tubular necrosis in patients with acute kidney injury.
Lab Values
Results
FeNa = (UNa × SCr) / (SNa × UCr) × 100
FeNa < 1%: Prerenal (kidney reabsorbing sodium)
FeNa > 2%: Intrinsic renal (ATN, tubular damage)
FeNa 1–2%: Indeterminate
The FeNa calculator computes the Fractional Excretion of Sodium, a clinical tool used to differentiate between prerenal azotemia and acute tubular necrosis (ATN) in patients presenting with acute kidney injury. A FeNa below 1% suggests the kidneys are appropriately retaining sodium (prerenal cause), while a FeNa above 2% suggests intrinsic renal damage.
FeNa can be unreliable in patients on diuretics (which increase urine sodium), in contrast nephropathy, myoglobinuria, and in patients with chronic kidney disease. In diuretic use, the Fractional Excretion of Urea (FeUrea) may be more reliable.
In a healthy person with normal kidney function, FeNa is typically around 1%. In acute kidney injury, FeNa <1% suggests prerenal causes (dehydration, heart failure), while FeNa >2% suggests intrinsic renal damage (ATN).