Following surgery a patient develops oliguria. You believe the oliguria is due to hypovolemia, but you seek corroborative data before increasing intravenous fluids. Which of the following values suppos the diagnosis of hypovolemia?

A fractional excretion of sodium (FENa) is calculated as (urine sodium x serum creatinine) / (serum sodium x urinary creatinine) x 100. A FENa less than 1% suppos a prerenal etiology for the patient's oliguria. When oliguria occurs postoperatively, it is impoant to differentiate between low output caused by the physiologic response to intravascular hypovolemia and that caused by acute tubular necrosis. A FENa of less than 1% in an oliguric setting indicates aggressive sodium reclamation in the tubules. Values above this suggest a tubular injury such that Na cannot be appropriately reclaimed. 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 patient, the urine osmolality would be over 500 mOsm/kg, the urine/serum creatinine ratio would be above 20, and the blood urea nitrogen (BUN)/creatinine ratio would be above 20.