Acute kidney injury represents an abrupt and sustained decrease in kidney function. It manifests as a decrease in urine output followed by an increase in serum creatinine.
Most definitions of AKI are based on these 2 variables, serum creatinine and urine output, and for a long time, there were many different definitions of AKI that complicated the comparison of studies and hampered clinical research.
The RIFLE (risk, injury, failure, loss of function and end-stage kidney disease) criteria have emerged as a commonly accepted way to define and grade the severity of AKI in the ICU(Fig. 1). Another commonly used definition was derived from the observation that even smaller changes in serum creatinine, for example, after cardiac surgery, affect outcome .
The Acute Kidney Injury Network (AKIN) definition defines AKI as an abrupt (within 48 hours) reduction in kidney function with an absolute increase in serum creatinine ≥0.3 mg/dL (≥26.4 μmol/L) or ≥50% (1.5-fold from baseline), or a reduction in urine output less than 0.5 mL/kg per hour for more than 6 hours [4].
Both definitions have limitations, and a third definition proposed more recently by the Kidney Disease Improving Global Outcomes group aims to combine the advantages of RIFLE
and AKIN definitions .
KIDGO defined AKI as either an increase in serum creatinine by 0.3 mg/dL within 48 hours or an increase of serum creatinine by 1.5 times baseline, which is known or presumed to have occurred within the prior 7 days or a decrease of urine volume to less than 0.5 mL/kg per hour for 6 hours. The group further defined 3 stages of AKI:
stage 1 is defined as either an increase of serum creatinine by 1.5 to 1.9 times or by 0.3mg/dL above baseline or a decrease of urine output to less than 0.5mL/kg per hour for 6 to 12 hours.
Stage 2 is defined as an increase of serum creatinine by 2.0 to 2.9 times baseline or a decrease of urine output to less than 0.5 mL/kg/h for more than 12 hours.
Stage 3 is defined by a tripling of serum creatinine or an increase of serum creatinine to more than 4.0 mg/dL or the initiation of renal replacement therapy or anuria for more than 12 hours