Fractional excretion of sodium (FENa) and urea (FEUrea)

Fractional excretion of sodium compares sodium clearance with creatinine clearance. In oliguric AKI a low value suggests intact tubular function (prerenal); fractional excretion of urea is less affected by diuretics.

What goes into the FENa and FEUrea

Serum sodium (mEq/L)
Serum creatinine (mg/dL)
Urine sodium (mEq/L)
Urine creatinine (mg/dL)
BUN (optional, for FEUrea) (mg/dL)
Urine urea nitrogen (optional) (mg/dL)

How to interpret the result

FENa < 1%: suggests prerenal physiology
FENa > 2%: suggests intrinsic tubular injury (ATN)
FEUrea < 35%: suggests prerenal; > 50% suggests intrinsic injury

Clinical pearls

Diuretics raise FENa; use FEUrea instead.
Low FENa also occurs in contrast nephropathy, rhabdomyolysis, heart failure, cirrhosis and hepatorenal syndrome.
Most useful in oliguric AKI; interpret with the clinical picture and urine sediment.

Evidence

Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236:579-581.

Related workups

Acute kidney injury Decompensated cirrhosis (ascites, SBP, hepatic encephalopathy)Rhabdomyolysis

Related calculators

KDIGO AKI stage Creatinine clearance (Cockcroft-Gault)All calculators

For education only. Check the inputs and the population the score was validated in before acting on it. Not a substitute for clinical judgment.