Acute Kidney Injury (AKI) Criteria
Acute Kidney Injury (AKI) is defined by any of the following three criteria: 1) increase in serum creatinine by ≥0.3 mg/dL (26.5 μmol/L) within 48 hours; 2) increase in serum creatinine to ≥1.5 times baseline within the prior 7 days; or 3) urine output <0.5 mL/kg/h for 6 hours or more. 1
Definition and Classification
AKI is classified into three stages of severity according to the Kidney Disease Improving Global Outcomes (KDIGO) guidelines:
Stage 1
- Increase in serum creatinine by 1.5-1.9 times baseline; OR
- Increase in serum creatinine by ≥0.3 mg/dL (26.5 μmol/L); OR
- Urine output <0.5 mL/kg/h for 6-12 hours
Stage 2
- Increase in serum creatinine by 2.0-2.9 times baseline; OR
- Urine output <0.5 mL/kg/h for ≥12 hours
Stage 3
- Increase in serum creatinine by ≥3.0 times baseline; OR
- Increase in serum creatinine to ≥4.0 mg/dL (353.6 μmol/L); OR
- Initiation of renal replacement therapy; OR
- In patients <18 years, decrease in eGFR to <35 mL/min/1.73 m²; OR
- Urine output <0.3 mL/kg/h for ≥24 hours; OR
- Anuria for ≥12 hours
Evolution of AKI Criteria
The current KDIGO criteria represent a harmonization of previous definitions:
RIFLE criteria (Risk, Injury, Failure, Loss, End-stage): Defined AKI based on a 50% increase in serum creatinine occurring over 1-7 days or oliguria for >6 hours 1
AKIN criteria (Acute Kidney Injury Network): Added an absolute increase in serum creatinine of 0.3 mg/dL and reduced the timeframe for creatinine increase to 48 hours 1
KDIGO criteria: Combined elements from both RIFLE and AKIN, maintaining the absolute increase in serum creatinine of 0.3 mg/dL within 48 hours from AKIN while incorporating the 7-day timeframe from RIFLE 1
Importance of Baseline Creatinine
Determining the baseline creatinine is crucial for accurate AKI diagnosis:
- A serum creatinine value obtained in the previous 3 months can be used as baseline 1
- When multiple values are available within the previous 3 months, the value closest to admission should be used 1
- In patients without a previous creatinine value, the admission creatinine may be used 1
Clinical Implications of AKI Staging
The staging of AKI has important prognostic implications:
- Higher AKI stages correlate with worse clinical outcomes, including mortality, need for renal replacement therapy, and progression to chronic kidney disease 2
- Even small increases in serum creatinine (Stage 1 AKI) are independently associated with increased mortality 1
- All stages of AKI are independently associated with six-month mortality, with hazard ratios increasing with each stage 3
Urine Output Criteria
Including urine output criteria in AKI diagnosis has significant implications:
- Using both serum creatinine and urine output criteria can nearly double the detected incidence of AKI compared to using serum creatinine alone 4
- Urine output criteria typically detect AKI earlier (median 13 hours after admission) compared to serum creatinine criteria (median 24 hours) 4
- Patients who fulfill both serum creatinine and urine output criteria have higher mortality compared to those who fulfill only one criterion 3
Special Considerations
AKI in Cirrhosis
In patients with cirrhosis, the International Club of Ascites modified the KDIGO criteria:
- Urine output criteria are removed as cirrhotic patients are often oliguric with preserved GFR or may have increased urine output due to diuretics 1
- Baseline creatinine can be from the previous 3 months rather than just 7 days 1
Limitations of Percentage-Based Criteria
The percentage changes in serum creatinine after severe AKI are highly dependent on baseline kidney function:
- With normal baseline function, a 90% reduction in creatinine clearance results in a 246% rise in serum creatinine at 24 hours
- In stage 4 CKD, the same reduction in clearance results in only a 47% rise in creatinine
- However, the absolute increase remains nearly identical (1.8-2.0 mg/dL) across the spectrum of baseline kidney function 5
Follow-up After AKI
Patients should be evaluated 3 months after an AKI episode to assess for:
- Resolution of kidney injury
- New onset of chronic kidney disease
- Worsening of pre-existing chronic kidney disease 1, 2
This follow-up is critical as AKI is associated with increased risk of developing chronic kidney disease, even after apparent recovery of kidney function.