Oliguria is the sudden decrease in urine output – most often defined as <400 mL/day or <0.5 mL/kg/h. It may be secondary to a multitude of factors. At the nephron level, acute oliguria occurs in the setting of a decreased glomerular filtration rate (GFR). With a drop in GFR, the kidney signals for sodium and water retention, leading to resultant hyperkalemia and acidosis. Additionally decreased excretion of creatinine and blood urea nitrogen occurs due to the lower filtration rate. When oliguria progresses to acute renal failure (ARF), the postsurgical patient has an approximate 50 % mortality rate, most commonly from sepsis or multiorgan failure. Oliguria has progressed to ARF when the glomerular filtration rate drops from greater than 100 mL/min to less than 30 mL/min.