AKI has historically been defined as a small elevation in serum creatinine, but new biomarkers are under investigation as rapidly obtainable and more accurate predictors of AKI [
25]. Twenty-four-hour urine creatinine clearance is a more accurate test for AKI than a rise in serum creatinine, but it is difficult to obtain in routine clinical practice. A rise in serum creatinine from baseline has therefore been considered to be a reasonable substitute. Trying to standardize AKI using serum creatinine has however serious flaws because creatinine is affected by factors other than AKI like volume overload, nutrition, steroids, and muscle trauma. Several biomarkers for AKI like tissue inhibitor of metalloproteinase-2, insulin-like growth factor binding protein-7, and neutrophil gelatinase-associated lipocalin have the potential to substantially improve the diagnostic approach to AKI in the future [
26,
27]. AKI is defined differently by different organizations. The Kidney Disease: Improving Global Outcomes (KDIGO) definition [
28] differs from the Risk, Injury, Failure, Loss, and End-Stage Renal Disease (RIFLE), the AKI Network (AKIN), and the Society of Thoracic Surgeons (STS) criteria [
29]. RIFLE uses Glomerular Filtration Rate (GFR) criteria in addition to serum creatinine and urine output criteria. GFR criteria were abandoned in the AKIN and KDIGO definitions. The Risk, Injury, and Failure categories of the RIFLE definition were replaced by AKIN and KDIGO stages 1, 2, and 3. In AKIN stage 1, an absolute rise in serum creatinine of more than 26.4 μmol/l was added to the relative increase of 150–200% in serum creatinine compared to baseline. This increase in serum creatinine of more than 26.4 μmol/l in AKIN stage 1 was replaced by an absolute rise in serum creatinine of more than 26.5 μmol/l in stage 1 of the KDIGO definition. Both in the RIFLE and KDIGO definitions, the increase in creatinine is defined to occur within 7 days, which contrasts with the 48 h used in the AKIN definition. In stage 3 of the AKIN and KDIGO definitions, the need for renal replacement therapy (RRT) was added. The categories Loss and End-Stage Kidney disease or equivalent (outcome) categories from the RIFLE definition were removed from the AKIN and KDIGO definitions. Urine output criteria are similar in RIFLE, AKIN, and KDIGO definitions.
AKIN and KDIGO detect more AKI patients than the RIFLE criteria, and this explains the large heterogeneity in the literature [
30]. The current STS definition of acute renal failure is a threefold increase in serum creatinine, a creatinine greater than 4mg/dl, or the initiation of dialysis. This definition fails to identify the vast majority of patients with AKI [
31].