<prompt> Following surgery, a patient develops oliguria.. You believe the patient is hypovolemic, but you seek corroborative data before increasing intravenous fluids. The best data is?Urine sodium of 28 meqLUrine chloride of 15 meqLFractional excretion of sodium less than 1Urinel Serum creatinine ratio of 20  <response>: 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 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 pent, the serum osmolality would be over 500 mOsmkg, and the urineserum creatinine ratio would be above 40. 