RIFLE Criteria for Acute Kidney Injury
The RIFLE classification system defines acute kidney injury (AKI) through five progressive stages based on serum creatinine changes, GFR decline, and urine output, with the first three stages (Risk, Injury, Failure) representing increasing severity of acute dysfunction and the last two (Loss, End-stage) representing clinical outcomes. 1
The Five RIFLE Stages
Acute Dysfunction Stages
Risk (R):
- Serum creatinine increased to 1.5 times baseline (50% increase) 1
- OR GFR decrease >25% 1
- OR urine output <0.5 mL/kg/h for 6 hours 1
- The creatinine increase must be both abrupt (within 1-7 days) and sustained (≥24 hours) 1
Injury (I):
- Serum creatinine increased to 2.0 times baseline (100% increase) 1
- OR GFR decrease 50-75% 1
- OR urine output <0.5 mL/kg/h for 12 hours 1
Failure (F):
- Serum creatinine increased to 3.0 times baseline (200% increase) 1
- OR serum creatinine increase of ≥0.5 mg/dL to a value ≥4.0 mg/dL 1
- OR GFR decrease >75% 1
- OR urine output <0.3 mL/kg/h for ≥24 hours or anuria for ≥12 hours 1
Outcome Stages
Loss (L):
- Persistent acute renal failure requiring renal replacement therapy (RRT) for >4 weeks 1
End-stage (E):
- Need for RRT for >3 months 1
Clinical Significance and Validation
The RIFLE classification demonstrates a stepwise increase in mortality risk with worsening stages: patients at Risk have a relative risk of death of 2.40, Injury patients have RR of 4.15, and Failure patients have RR of 6.37 compared to non-AKI patients (P<0.0001 for all). 2
- This graded mortality relationship has been validated across more than 71,000 patients in multiple clinical settings 2
- The system successfully identifies that even mild degrees of kidney dysfunction (Risk stage) have significant negative impact on patient outcomes 2
Important Caveats and Limitations
RIFLE has been largely superseded by the KDIGO criteria in modern clinical practice, which harmonized elements from both RIFLE and the subsequent AKIN classification into a superior framework. 3, 4
Key limitations of RIFLE include:
- The GFR criteria are problematic because serum creatinine significantly overestimates actual kidney function in patients with muscle wasting, volume expansion, or hyperbilirubinemia 5
- Urine output criteria are unreliable in cirrhotic patients with ascites who may be oliguric despite maintaining relatively normal GFR 1, 5
- The system can miss patients with definite AKI depending on baseline creatinine availability and timing of measurements 6
- The 7-day timeframe for creatinine changes may delay diagnosis compared to the 48-hour window used in AKIN/KDIGO 7
In special populations (particularly cirrhosis with ascites): focus exclusively on serum creatinine changes rather than urine output, as diuretic therapy and sodium retention confound interpretation. 5, 4