Perioperative oliguria is common, is almost always prerenal, and rarely implies acute renal failure. BUN to serum creatinine ratio of greater than 20:1 implies the existence of a prerenal syndrome. A doubling of the serum creatinine implies a halving of the GFR. Ketoacidosis, barbiturates, and cephalosporin antibiotics may artifactually increase the serum creatinine level by as much as 100%. Serum creatinine concentration does not increase above the normal range until the GFR declines below 50 mL/min.