BACKGROUND: The acute reduction of kidney function in critically and noncritically ill patients (regardless of their age) is one of the deadliest clinical conditions ever reported in modern medicine. Acute kidney injury (AKI) symptoms are sneaky and potentially difficult to be identified at the right time at the bedside. One of the greatest efforts of the recent history of critical care nephrology has been to find a common classification for AKI definition and staging with the purpose of allowing a timely diagnosis and push forward epidemiologic research.
SUMMARY: AKI is currently defined by the Kidney Disease Improving Global Outcomes (KDIGO) consensus classification that applies conventional serum creatinine and urine output (UO) criteria. According to a recent large epidemiologic study, this classification led to the confirmation that AKI occurs in about half of adult critically ill patients admitted to the intensive care unit and that a stepwise increase in mortality is associated with the severity of AKI along KDIGO stages. Both serum creatinine and UO have inherent limitations in accurately diagnosing abrupt decreases of renal function, but their common and easy application in routine clinical practice is currently considered the standard of care for AKI diagnosis. Pediatric and neonatal AKI have recently been described and specific staging with KDIGO modification has been proposed. Key Messages: AKI is frequent in critically ill patients and significantly affects intensive outcomes independent of other clinical factors. AKI can be diagnosed and its severity accurately staged by the KDIGO classification and its modification for pediatric patients. Serum creatinine and UO criteria are applied in order to diagnose and stage AKI. Despite some significant limitations of these commonly applied biomarkers, their application has made it possible to clearly appraise the importance of accurate AKI identification in clinical practice in several studies for prognostic and therapeutic purposes.