For decades, nephrologists used the term acute renal failure (ARF), defined as a sudden (within hours to days) decrease in renal function, leading to retention of nitrogenous waste products (e.g., urea and creatinine). Classically, ARF was divided into prerenal failure (decreased renal blood flow leading to decreased glomerular filtration rate), postrenal failure (urinary obstruction), and intrinsic renal failure, due to injury to kidney tissue, most commonly tubules (acute tubular necrosis [ATN]) but also interstitial cells (acute interstitial nephritis [AIN]), and less commonly glomeruli (acute glomerulonephritis), or blood vessels (acute vasculopathy/vasculitis).

More recently, the term acute kidney injury (AKI) has been used instead of ARF or ATN. However, since not all patients with ARF have kidney injury, and there are no biochemical markers of kidney injury in widespread clinical use in the United States at this time, the term AKI as currently used is a bit of a misnomer.

Several consensus definitions of AKI have been developed in an attempt to provide a uniform definition. In 2004, the Acute Dialysis Quality Initiative (ADQI) group, which included expert intensivists as well as nephrologists, proposed a consensus graded definition, called the RIFLE criteria (Bellomo et al., 2004). A modification of the RIFLE criteria was subsequently proposed by the Acute Kidney Injury Network (AKIN) (Mehta et al., 2007). More recently, the Kidney Disease: Improving Global Outcomes (KDIGO) AKI Workgroup also proposed a definition (KDIGO, 2012) (Table 10.1).

According to AKIN diagnostic criteria for AKI, an increase in the serum creatinine concentration of as little as 0.3 mg/dL (26.4 µmol/L)
from baseline, or a 1.5-fold or greater increase in serum creatinine over 48 hours, or oliguria (<0.5 mL/kg/h) for more than 6 hours is sufficient to establish the diagnosis. However, these criteria should be applied only after volume status had been optimized and urinary tract obstruction excluded. KDIGO added the modification that the 1.5-fold increase in serum creatinine could develop over less than 7 days instead of less than 48 hours. According to KDIGO, AKI can include various etiologies of acute renal dysfunction, including prerenal azotemia, ATN, AIN, acute glomerular and vasculitic renal diseases, and acute postrenal obstructive nephropathy. The inclusion of very small increments in serum creatinine in the diagnostic criteria was based on epidemiologic data, showing that such patients have worse outcomes (Newsome et al., 2008)