Proposed classification scheme for ARF. The classification system includes separate criteria for creatinine and urine output (UO). A patient can fulfil the criteria through changes in serum creatinine (SCreat) or changes in UO, or both. The criteria that lead to the worst possible classification should be used. Note that the F component of RIFLE (Risk of renal dysfunction, Injury to the kidney, Failure or Loss of kidney function, and End-stage kidney disease) is present even if the increase in SCreat is under threefold, as long as the new SCreat is greater than 4.0 mg/dl (350 μmol/l) in the setting of an acute increase of at least 0.5 mg/dl (44 μmol/l). The designation RIFLE-F_C should be used in this case to denote 'acute on chronic disease'. Similarly, when the RIFLE-F classification is achieved by UO criteria, a designation of RIFLE-F_O should be used to denote oliguria. The shape of the figure highlights the fact that more patients (high sensitivity) will be included in the mild category, including some who do not actually have ARF (less specificity). In contrast, at the bottom of the figure the criteria are strict and therefore specific, but some patients will be missed. ARF, acute renal failure; GFR, glomerular filtration rate.