Question: “Can you help with the definition of acute renal failure. I know in the last few years I have seen greater use of the term, acute kidney injury and there are more than one set of guidelines. It would be nice to have a simple explanation that one could remember while doing those 3am admits.”
The other day I received this question in an email from a colleague. This is a great question but the truth is that there is not a whole lot of consensus even amongst the nephrologists about the definition of acute kidney injury.
Traditionally, acute renal failure (ARF) has been broadly described as a sudden decline in kidney function leading to the accumulation of urea and other nitrogenous waste products and the disturbances in volume, electrolytes and acid-base balance. The lack of a universally recognized definition of ARF has posed a significant limitation in early and appropriate diagnosis and even in interpreting epidemiological studies.
To resolve these limitations, the Acute Dialysis Quality Initiative (ADQI) proposed the RIFLE criteria. This consists of three graded levels of injury (Risk, Injury, and Failure) based upon either the magnitude of elevation in serum creatinine or urine output, and two outcome measures (Loss and End-stage renal disease). More recently the Acute Kidney Injury Network (AKIN), modified the RIFLE criteria recognizing that even very small changes in SCr (≥0.3 mg/dL) adversely impact clinical outcome. In addition, the term acute kidney injury (AKI) was proposed to represent the entire spectrum of acute renal failure from subclinical disease to complete organ failure. They defined AKI as
‘an abrupt (within 48 hours) absolute increase in the serum creatinine concentration of ≥0.3 mg/dL (26.4 micromol/L) from baseline, a percentage increase in the serum creatinine concentration of ≥50 percent, or oliguria of less than 0.5 mL/kg per hour for more than six hours.’
The staging system for AKI is comprised of three stages of increasing severity, which correspond to risk (stage 1), injury (stage 2), and failure (stage 3) of the RIFLE criteria (see Table)
The above criteria require at least two creatinine values within 48 hours. It is assumed that the diagnosis based on the urine output criterion alone will require exclusion of urinary tract obstructions or other easily reversible causes of reduced urine output. The above criteria should be used in the context of the clinical presentation and following adequate fluid resuscitation.
The RIFLE and AKI criteria have correlated with prognosis in a number of studies. But, there is an inherent confusion within these criteria as to whether prerenal and obstructive etiologies of ARF are included in the definition of AKI. The exact clinical utility of these criteria are yet to be proven and in the coming years may be revised.
So, while there is no simple definition of AKI, here’s the bottom line:
1. Acute kidney injury (AKI) has replaced the term Acute Renal Failure (ARF)
2. AKI is defined as an abrupt (within 48 hours):
- absolute increase in the serum creatinine concentration of ≥0.3 mg/dL (26.4 micromol/L) from baseline,
- ≥50 percent increase in the serum creatinine or,
- oliguria of less than 0.5 mL/kg per hour for more than six hours.