Reflections on the KDIGO Definition of Acute Kidney Injury and Its Integration in the Concept of Acute Diseases and Disorders and Chronic Kidney Diseases

Acute kidney injury (AKI) describes a heterogeneous group of conditions, without specification of their etiology and diagnosed only by indirect markers of glomerular filtration rate (GFR), such as serum creatinine and urine output. Bedside estimation of GFR and detection of structural alterations with novel biomarkers, and stress tests have more recently been developed. These novel findings should probably be included in future AKI definitions. Chronic kidney disease (CKD) is defined by abnormalities in kidney function and structure that persist over >3 months and is classified according to cause, GFR, and albuminuria. Acute kidney disease (AKD) is the term representing patients with abnormalities of function and structure with a duration of <= 3 months that fall outside the definitions of AKI or CKD. Since AKI is by definition also AKD, 2 types of AKD have been proposed, one with and one without AKI. AKD without AKI is common, often undetected, occurs frequently in the outpatient population and shows increased risk of CKD, ESKD and mortality. Alternatively, AKD has also been defined as the period of incomplete recovery following an AKI episode, the latter limited for the duration of 7 days. This contribution discusses the pros and cons of the existence of these 2 definitions of AKD.