The definitions and staging systems of acute kidney injury and their limitations in practice

It is now evident that even minimal reductions in glomerular filtration rate (GFR) are associated with a dramatic increase in mortality. The term acute kidney injury (AKI) describes an acute fall of estimated GFR (eGFR) and allows patient stratification based on AKI severity.

The Risk, Injury, Failure, Loss and End-stage kidney disease (RIFLE) system defines AKI by a change in serum creatinine (SCr) level or eGFR from a baseline value, and urine output per kilogram of body weight over a specified time period. The Acute Kidney Injury Network (AKIN) definition was based on the RIFLE system but added an absolute change in SCr of ≥ 0.3 mg/dL, omitted eGFR criteria and included a time constraint of 48 hours. The AKIN system also omitted the stages "Loss" and "End-stage" and allocated patients who needed acute dialysis to stage-3. The most recent Kidney Disease Improving Global Outcomes (KDIGO) guidelines retained the AKIN staging criteria but allowed a time frame of seven days for a 50% increase in SCr. The KDIGO criteria do not rely on changes in GFR for staging except in children under the age of 18 years. AKI misclassification may result from the lack of a uniform approach to estimate baseline SCr and the changes in SCr concentrations resulting from acute severe illness and altered fluid balance. In addition, exact data on urine output are not always available resulting in underutilization of the urine output criteria.

The existing definitions of AKI rely on imperfect markers of renal function rather than direct measures of kidney damage, but remain an important diagnostic and prognostic tool.