Acute renal failure is a syndrome of sudden diminution or cessation of kidney function. Despite the significant morbidity and mortality associated with acute renal failure, over 30 definitions exist in the published literature.[1] Definitions vary according to specific patient populations and the outcome measure (eg, rise in creatinine versus need for renal replacement therapy). Given that most acute renal failure definitions are based on a serum creatinine rise, lack of a uniform definition may result in lack of recognition of significant kidney injury and delay in treatment. In addition, creatinine-based acute renal failure definitions are problematic for infants and small children because their normal serum creatinine level is 0.2 to 0.4 mg/dL. As a result, a serum creatinine change of 0.1 mg/dL represents a 25% to 50% change for small children.

A graded acute renal failure classification system, which identifies patients at risk for developing significant kidney insult and metabolic disturbance, is needed to assess best the state of kidney injury in patients with a renal insult. One such system uses the RIFLE criteria (risk, injury, failure, loss and end-stage renal disease) to classify the degree of kidney insult by changes in serum creatinine and the duration of decreased urine output.[1] Furthermore, a change in terminology from acute renal failure to acute kidney injury (AKI) focuses attention on early recognition of kidney insult and interventions to prevent or mitigate the effects of significant renal failure.

Until RIFLE or other AKI classification systems undergo systematic validation, clinicians still rely on historical definitions based on urine output to characterize the degree of acute renal failure. For the purposes of this chapter the following definitions are used for decreased urine output:

• Anuria: less than 100 mL/m[2]/day
• Oliguria: less than 300 mL/m[2]/day
• Nonoliguric state: sufficient urine volume to allow for administration of necessary fluids, nutrition, blood products, and medication without resulting volume overload
• Oligoanuric state: insufficient urine volume to allow for administration of necessary fluids, nutrition, blood products, and medication without resulting volume overload