<prompt> 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?Urine sodium of 28 mEqLUrine chloride of 15 mEqLFractional excretion of sodium less than 1Urineserum creatinine ratio of 20  <response>: 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 patients 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 kidneys effos to retain volume: the urine sodium would be below 20 mEqL, the urine chloride would not be helpful except in the metabolically alkalotic patient, the urine osmolality would be over 500 mOsmkg, the urineserum creatinine ratio would be above 20, and the blood urea nitrogen BUNcreatinine ratio would be above 20. 