When oliguria occurs postoperatively, it is impoant to differentiate between low output caused by the physiologic response to intravascular hypovolemia and that caus ed by acute tubular necrosis. The fractional excretion of sodium (FENa) is an especially useful test to aid in this differentiation. Values of FE < 1% in an oliquric setting indicate aggressive sodium reclamation in the tubules; values above this suggest tubular injury. Calculating the fractional excretion is simple: (urine Na x serum creatinine) + (serum sodium x urinary creatinine). In the setting of postoperative hypovolemia, all findings would reflect the kidney's effos to retain volume: the urine sodium would be below 20 meq/l, the urine chloride would not be helpful except in the metabolically alkalotic pent, the serum osmolality would be over 500 mOsm/kg, and the urine/serum creatinine ratio would be above 40.