Question 12

Describe the RIFLE classification system for Acute Kidney Injury and briefly discuss its implications and limitations.

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College Answer

AKI can be defined as an abrupt (1 to 7 days) and sustained (more than 24 hours) decrease in kidney function. The ADQI formulated the RIFLE criteria to allow for AKI to be objectively and uniformly defined.

RIFLE classification of acute kidney injury

The implication of this classification is that a progression down the RIFLE criteria is associated with a higher length of stay in ICU and Hospital and is associated with a higher mortality.

The limitations of this classification relate to it’s dependence on measuring urine output and creatinine which is confounded by the following:-

  1. Accuracy of urine output measures.
  2. Urine output affected by the use of diuretics.
  3. Baseline creatinine may be affected by the patients concurrent health problem eg it may be falsely high purely because the patient was dehydrated on admission.
  4. It is uncertain how well balanced urine output and creatinine are even though they have been given an equal weighting.


This question refers to the 2004 formulation of the RIFLE criteria by the ADQI.

 This is a system of classification which was supposed to step in and define the seveirty of renal failure, at a time when there were over 30 different disagreeing definitions. Its real use is in predicting mortality, and it helps decisionmaking in which mortality statistics play a role.

The Acute Dialysis Quality Initiative formulated this classification. Our very own Ronco, Bellomo and Kellum published this thing in 2004. And of course, its varios advantages and disadvantages have been picked apart in the literature reviews, not the least of which comes from Life In The Fast Lane. ADQI themselves have also addressed the limitations of their system.

The table itself is quoted in the college answer:

Risk Creatinine x 1.5 u/o < 0.5ml/kg/hr x 6 hrs
Injury Creatinine x 2 u/o < 0.5ml/kg/hr x 12 hrs
Failure Creatinine x 3 u/o < 0.3ml/kg/hr x 24 hrs
Loss Complete loss of function > 4 weeks  
End-stage Complete loss of function > 3 months  

Major complaints against this system are elaborated upon elsewhere.

In brief:

  • The system classifies renal failure once it has already become established.
  • The criteria are unbalanced: GFR and urine criteria for the same class (R) have different outcomes, but have been given equal weighting.
  • Urine criteria rely on impractical 6th and 12th hour measurements.
  • Urine output criteria are altered by diuretics, DI, etc.
  • Urine output measurements may be inaccurate.
  • Creatinine levels may be affected by factors which do not affect outcome.
  • The criteria for creatinine rely on the availability of baseline values.
  • Nobody seems to agree on what the definiton of "baseline" is, anyway.
  • The "Risk" criteria for creatinine may not be sensitive enough.
  • The criteria require true GFR, rather than an estimated GFR.
  • The criteria make no attempt to distinguish between different aetiologies.

The greatest criticism of this system would come from the front lines of critical care, where the definitions and their prognostic value play little role. Sure, they have some utility in epidemiology, but at the bedside there is no way to apply these criteria to make management decisions. The fact that only 2% of the candidates passed this question is telling. Cold pragmatic bastards, they never saw a point in studying something they all viewed as a research tool.