From the Guidelines
Acute Kidney Injury (AKI) is classified into three stages based on the KDIGO criteria: Stage 1, mild injury with serum creatinine rising 1.5-1.9 times baseline or by 0.3 mg/dL, or urine output below 0.5 mL/kg/hr for 6-12 hours; Stage 2, moderate injury with creatinine 2.0-2.9 times baseline or urine output below 0.5 mL/kg/hr for over 12 hours; and Stage 3, severe injury with creatinine 3 times baseline or above 4.0 mg/dL, urine output below 0.3 mL/kg/hr for 24+ hours, or anuria for 12+ hours, as defined by the most recent study 1.
The classification of AKI is crucial in guiding treatment approaches and predicting outcomes. The KDIGO criteria provide a standardized framework for diagnosing and staging AKI, which is essential for clinicians to identify patients at risk of developing chronic kidney disease (CKD) or end-stage renal disease (ESRD) 1.
Key characteristics of each stage of AKI include:
- Stage 1: Increase in serum creatinine (SCr) by 0.3 mg/dL within 48 hours, or increase in SCr to 1.5 times baseline, or urine output less than 0.5 mL/kg/h for 6-12 hours 1.
- Stage 2: Increase in SCr to 2.0-2.9 times baseline, or urine output less than 0.5 mL/kg/h for over 12 hours 1.
- Stage 3: Increase in SCr to 3.0 times baseline, or increase in SCr to 4.0 mg/dL, or initiation of renal replacement therapy, or urine output less than 0.3 mL/kg/h for 24 hours, or anuria for 12 hours 1.
It is essential to note that the KDIGO criteria also emphasize the importance of evaluating patients 3 months after AKI for resolution, new onset, or worsening of pre-existing CKD, and managing them accordingly 1.
In clinical practice, the KDIGO classification system has been widely adopted, and its use has been shown to improve patient outcomes by facilitating early recognition and treatment of AKI 1.
Overall, the KDIGO criteria provide a comprehensive framework for diagnosing and managing AKI, and their use is recommended in clinical practice to improve patient outcomes and reduce the risk of CKD and ESRD 1.
From the Research
Definition and Staging of Acute Kidney Injury (AKI)
The definition and staging of AKI are mainly based on the Risk, Injury, Failure, Loss, End-stage kidney disease (RIFLE) criteria, the Acute Kidney Injury Network (AKIN) criteria, and the Kidney Disease Improving Global Outcomes (KDIGO) guidelines 2, 3.
- The RIFLE system defines AKI by a change in serum creatinine (SCr) level or estimated GFR (eGFR) from a baseline value, and urine output per kilogram of body weight over a specified time period 2.
- The AKIN definition is based on the RIFLE system but adds an absolute change in SCr of ≥ 0.3 mg/dL, omits eGFR criteria, and includes a time constraint of 48 hours 2.
- The KDIGO guidelines retain the AKIN staging criteria but allow a time frame of seven days for a 50% increase in SCr 2.
Levels of Acute Kidney Injury (AKI)
The levels of AKI can be classified into three categories:
- Prerenal: caused by poor renal perfusion or decreased blood flow to the kidneys 4, 5, 3.
- Intrinsic renal: caused by direct damage to the kidneys, such as acute tubular necrosis 5.
- Postrenal: caused by obstruction of the urinary tract 4, 5, 3. The KDIGO guidelines also define three stages of AKI based on the increase in SCr level and urine output:
- Stage 1: increase in SCr by 0.3 mg/dL or more within 48 hours, or urine output less than 0.5 mL/kg/h for 6-12 hours 6.
- Stage 2: increase in SCr to 2.0-2.9 times the baseline, or urine output less than 0.5 mL/kg/h for more than 12 hours 6.
- Stage 3: increase in SCr to 3.0 times the baseline or more, or urine output less than 0.3 mL/kg/h for 24 hours or more, or anuria for 12 hours or more 6.
Limitations of Current Definitions
The current definitions of AKI have some limitations, including 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 2. Additionally, exact data on urine output are not always available, resulting in underutilization of the urine output criteria 2.