Definitions, classification, epidemiology, and risk factors of acute kidney injury

The lack of a precise biochemical definition of acute kidney injury (AKI) resulted in at least 35 definitions in the medical literature, which gave rise to a wide variation in reported incidence and clinical significance of AKI, impeded a meaningful comparison of studies.The first part of this chapter describes and discusses different definitions and classification systems of AKI. Patient outcome and the need for renal replacement therapy are directly related to the severity of AKI, an observation that supports the use of a categorical staging system rather than a simple binary descriptor. The severity of AKI is commonly characterized using the relative changes in serum creatinine and urine output. Recently introduced staging systems including the RIFLE classification and the Acute Kidney Injury Network (AKIN) use these relatively simple and readily available parameters allowing the assignment of individual patients to different AKI stages. More recently, a Kidney Disease: Improving Global Outcomes (KDIGO) workgroup developed a consensus-based AKI staging system drawing elements of both RIFLE and AKIN. The potential pitfalls and limitations of the proposed definitions and classifications are briefly described.The second part of the chapter describes the epidemiology of AKI in different clinical settings; the intensive care unit (ICU), the hospitalized population, and the community. The different spectrum of AKI in the emerging countries is discussed and the most important causes and aetiologies of the major clinical types of AKI, prerenal, renal, and post-renal are summarized in table form. Finally the patient survival and renal functional outcome of AKI are briefly discussed