The AKIN classification system is about as old as the RIFLE system, but has not been the focus of any CICM exam questions. It was first promoted in a March (2007) issue of Critical Care.
The AKIN system
The classification system separates renal failure into 3 categories:
|Serum creatinine||Urine output criteria|
|Creatinine × 1.5 - 2.0 from baseline
Creatinine increased by at least 26.4μmol/L
|u/o < 0.5ml/kg/hr × 6 hours|
|Creatinine × 2.0-3.0 (i.e. doubled or tripled creatinine)||u/o < 0.5ml/kg/hr × 12 hrs|
|Creatinine > 3.0
A creatinine over 354 μmol/L, with an acute increase by at least 44 μmol/L
The initiation of RRT
|u/o < 0.3ml/kg/hr × 24 hrs
Anuria for 12 hours
This is a modification and an improvment of the RIFLE system, created as an answer to the various criticisms of the RIFLE criteria. Thus, in order to appreciate this system, one ought to first have knowledge of the RIFLE classification, and its various shortcomings.
The major changes are:
- The diagnosis of AKI is only considered after achieving an adequate status of hydration and after
excluding urinary obstruction.
- The AKIN classiﬁcation only relies on serum creatinine, not GFR.
- Baseline creatinine is not necessary
- Requires at least two creatinine measurements, within the same 48-hour period
Thus, Stage 1 corresponds to the "Risk" class; Stage 2 to the "Injury" class and Stage 3 to the "Failure" class. The "Loss" and "ESRD" categories were dumped.
Advantages over the RIFLE classification:
Lopes et al have published "a critical and comprehensive review" of the RIFLE and AKIN criteria in 2012. This summary owes much to their article.
Advantages of the AKIN system over the RIFLE system:
- The AKIN score is independently associated with outcome. Just as RIFLE; so this is not really an advantage; rather a demonstration of non-inferiority.
- The score is taken after hypovolemia has been corrected, thus excluding a group who were merely dehydrated. This introduces a certain aetiological selectivity into the process.
- The score demands that urinary tract obstruction is excluded. This way urine output criteria retain their meaning, and again a certain spectrum of the AKI aetiology is controlled for.
- The GFR is discarded, killing the controversy of how one should measure it. Previous criticisms of the RIFLE criteria have made much of the fact that a "true" GFR should be used, rather than an estimated GFR (eGFR).
- Baseline creatinine is not required (though two measurements within the same 48 hour period are necessary). The baseline creatinine is replaced by the first "reference" reading.
- There is a defined timeframe over which the creatinine is measured, this being 48 hours. This gives a firm framework for the rate of progression in acute renal failure.
- A small absolute increase in creatinine will still be detected as "Stage 1" (i.e. the system is more sensitive; it is known that patients with a mild creatinine elevation still had increased mortality increases in creatinine smaller than the RIFLE-specified factor of 1.5 are still associated with poorer outcome).
- The AKIN classiﬁcation could theoretically improve the RIFLE criteria sensitivity and specificity, although the advantages of the RIFLE modiﬁcations have not been proven. In fact it turns out that AKIN criteria identify more patients with acute kidney injury, but do not improve our ability to predict in-hospital mortality.
Criticisms of the AKIN system:
Those demerits which are unique to the AKIN system are as follows:
- Creatinine has to be measured within a 48 hour window, which may exclude more slowly progressive aetiologies. Renal failure which may develop over some weeks still remains undetected by this scoring system.
- Patients who are subjected to RRT are scored as Stage 3, irrespective of the indication for RRT (eg. perhaps it is being used to remove some sort of toxin, and not because of renal failure?)
- There is a significant variability in local practice regarding commencement of RRT. This could negatively influence the prognostic value of "Stage 3".
- Patients are scored only after an "optimal state of hydration" is achieved, but this is an ill-defined state, and many may disagree as to what this actually means. "Optimal state" could be defined in a number of ways and by a whole series of different parameters.
- Probably the strongest criticism of the AKIN modification to the RIFLE system is the fact that there is no difference in predictive value between these systems. Lopes and Jorge present six different studies which all failed to demonstrate a greater prognostic acuity for AKIN over RIFLE.
The following criticisms are shared by the RIFLE and the AKIN systems:
- The system classifies renal failure once it has already become established, which is perhaps too late.
- The criteria are unbalanced in outcome, but equal in weighting. The creatinine (GFR) criteria and the urine criteria are weighed the same, but have different associations with mortality. For instance, patients stratified into the "Risk" (or "Stage 1") class by the creatinine criteria are more severely ill than those stratified into the same class by urine output criteria.
- Urine criteria rely on impractical measurements: The system requires 6th hour and 12th hour urine measurements, which are not routinely collected. This makes retrospective data collection difficult.
- Urine output criteria are altered by diuretics, DI, etc.- i.e. they may not be a brilliantly accurate reflection of renal function.
- Urine output measurements may be inaccurate. A mildly unwell "Risk" or "Stage 1" class patient may have no catheter, and may be voiding in an uncontrolled manner all over the hospital bathroom or bedsheets.
- The "Risk" criteria for creatinine may not be sensitive enough: in one study y, but did not qualify for the "Risk" category.
- The criteria require true GFR, rather than an estimated GFR: using the eGFR is inherently invalid, because the MDRD equation used to calculate eGFR is valid only in steady-state conditions (and you with your failing kidneys are by definition not in a steady state).
- The criteria make no attempt to distinguish between different aetiologies; the differences may have powerful prognostic implications (eg. renal impairement due to dehydration vs. lupus nephritis).