RIFLE Criteria for Acute Kidney Injury Classification
Overview and Current Status
The RIFLE classification system defines acute kidney injury through five progressive stages: Risk (R), Injury (I), Failure (F), Loss (L), and End-stage (E), based on serum creatinine changes, GFR decline, and urine output criteria—though this system has been largely superseded by the KDIGO criteria in modern clinical practice. 1, 2
The Five RIFLE Stages
Stage 1: Risk (R)
- Serum creatinine: Increased to 1.5 times baseline 1
- GFR: Decrease >25% 1, 2
- Urine output: <0.5 mL/kg/h for 6 hours 1, 2
Stage 2: Injury (I)
- Serum creatinine: Increased to 2.0 times baseline 1
- GFR: Decrease of 50-75% 1, 2
- Urine output: <0.5 mL/kg/h for 12 hours 1, 2
Stage 3: Failure (F)
- Serum creatinine: Increased to 3.0 times baseline OR increase of ≥0.5 mg/dL to a value ≥4.0 mg/dL 1
- GFR: Decrease >75% 1, 2
- Urine output: <0.3 mL/kg/h for 24 hours OR anuria for 12 hours 1, 2
Stage 4: Loss (L)
- Definition: Persistent acute renal failure requiring renal replacement therapy (RRT) for >4 weeks 1, 2
Stage 5: End-stage (E)
Critical Application Rules
The patient should be classified according to the worst criterion met, whether creatinine, GFR, or urine output. 1 The increase in serum creatinine must be both abrupt (within 1-7 days) and sustained (≥24 hours). 1
When a patient reaches RIFLE-F classification through an acute-on-chronic presentation (creatinine >4.0 mg/dL with acute rise ≥0.5 mg/dL), use the designation "RIFLE-FC" to denote this pattern. 1 Similarly, when RIFLE-F is achieved by urine output criteria alone, designate as "RIFLE-FO" for oliguria. 1
Important Limitations and Caveats
When to Avoid Urine Output Criteria
In patients with cirrhosis and ascites, focus exclusively on serum creatinine changes rather than urine output. 2, 3 These patients are frequently oliguric with avid sodium retention despite maintaining relatively normal GFR, and diuretic therapy further confounds interpretation. 2, 3
GFR Criteria Problems
The GFR criteria are problematic because serum creatinine significantly overestimates actual kidney function in patients with muscle wasting, volume expansion, or hyperbilirubinemia. 2 Additionally, massive fluid resuscitation can dilute serum creatinine concentration, potentially masking significant GFR reduction. 3
Establishing Baseline Without Known Values
When no baseline creatinine is available, calculate a theoretical baseline using the simplified MDRD formula assuming a GFR of 75-100 mL/min per 1.73 m². 1 For example, a 50-year-old Black female would have an estimated baseline creatinine of 1.0 mg/dL. 1
Clinical Validation and Outcomes
Validation studies have demonstrated that increasing RIFLE severity correlates with progressively worse mortality rates. 1, 4 The system has been successfully applied across multiple clinical settings including critically ill patients, cardiac surgery patients, and emergency department presentations. 5, 6, 7
In pediatric populations, a modified version (pRIFLE) has been developed and validated, showing that 82% of AKI occurs within the first week of ICU admission and that AKI is an independent predictor of mortality with an odds ratio of 3.0. 8
Modern Practice Recommendation
The KDIGO criteria have superseded RIFLE in contemporary clinical practice, harmonizing elements from both RIFLE and the subsequent AKIN classification into a superior framework. 2, 9 The KDIGO nomenclature guidelines from 2020 explicitly state that previous classifications like RIFLE should be avoided in favor of the unified KDIGO system. 9
However, understanding RIFLE remains important for interpreting older literature and for contexts where KDIGO has not been fully implemented. 4