From the Guidelines
Acute kidney injury (AKI) is defined as any of the following: an increase in serum creatinine by ≥0.3 mg/dL within 48 hours; an increase in serum creatinine to ≥1.5 times baseline within 7 days; or urine volume reduction to <0.5 mL/kg/hour for 6 hours, as per the most recent and highest quality study 1.
Definition and Staging of AKI
The definition and staging of AKI are crucial for prompt identification and management of kidney injury. The most widely used criteria for defining AKI are based on the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines.
- The KDIGO criteria define AKI as any of the following:
- Increase in serum creatinine by ≥0.3 mg/dL within 48 hours
- Increase in serum creatinine to ≥1.5 times baseline within 7 days
- Urine volume reduction to <0.5 mL/kg/hour for 6 hours
AKI Severity Stages
AKI severity is further classified into three stages:
- Stage 1:
- Increase in serum creatinine by 1.5-1.9 times baseline
- Increase in serum creatinine by ≥0.3 mg/dL
- Urine output <0.5 mL/kg/hour for 6-12 hours
- Stage 2:
- Increase in serum creatinine by 2.0-2.9 times baseline
- Urine output <0.5 mL/kg/hour for ≥12 hours
- Stage 3:
- Increase in serum creatinine to 3.0 times baseline
- Increase in serum creatinine to ≥4.0 mg/dL
- Initiation of renal replacement therapy
- Urine output <0.3 mL/kg/hour for ≥24 hours
- Anuria for ≥12 hours These criteria help clinicians promptly identify kidney injury, allowing for early intervention to prevent further damage and complications, as supported by the study 1.
From the Research
Definition of Acute Kidney Injury (AKI) Values
The definition of AKI values is based on the increase in serum creatinine (SCr) or decrease in urine output. According to the study 2, the percentage changes in SCr after severe AKI are highly dependent on baseline kidney function.
- The Acute Dialysis Quality Initiative (RIFLE) defines AKI largely by percentage increases in SCr over baseline.
- The Acute Kidney Injury Network defines the first stage by either an absolute or a percentage increase in SCr.
- The Kidney Disease: Improving Global Outcomes (KDIGO) clinical practice guideline classifies AKI into 3 stages defined by serum creatinine elevation or urine output decline 3.
Criteria for Defining AKI Values
The criteria for defining AKI values include:
- Absolute increase in SCr: a 0.3 mg/dl increase in SCr within 48 hours 3.
- Percentage increase in SCr: a 50% relative increase in SCr within 7 days 3.
- Urine output criteria: a mean UO < 0.5 mL/kg/h over time 4.
- Novel UO-based AKI classification system: stage 1 (0.2-0.3 mL/kg/h), stage 2 (0.1-0.2 mL/kg/h), and stage 3 (< 0.1 mL/kg/h) over 6 hours 5.
Classification Systems
There are different classification systems for AKI, including:
- RIFLE criteria: defines AKI based on percentage increases in SCr over baseline 2.
- KDIGO criteria: classifies AKI into 3 stages defined by serum creatinine elevation or urine output decline 3, 4.
- Novel UO-AKI classification system: proposes a new classification system based on urine output thresholds 5.
Clinical Outcomes
The clinical outcomes of AKI are affected by the definition and classification of AKI. According to the study 3, patients with AKI stages 1a and 1b experienced clinically meaningful and statistically significant differences in length of stay and mortality. The study 5 found that the proposed UO-AKI classification system outperformed the KDIGO criteria in terms of predictive accuracy.