Fractional excretion of sodium (FENa) = (UNa x PCr) / (PNa x UCr) x 100
FENa < 1% =prerenal azotemia,
> 2% = intrinsic renal azoemia (eg, ATN)

UNa = urine sodium; (meq/l)
PCr = plasma creatinine; (mg/dl)
PNa = plasma sodium; (meq/l)
UCr = urine creatinine (mg/dl)
Result (%)