Acute kidney injury: We’ve got the diagnosis, now what?

In 2004, our understanding of the impact of kidney injury on patient outcome received a huge fillip with the publication of the Risk, Injury, Failure, Loss, and End-stage renal disease (RIFLE) criteria. A multinational group of experts calling themselves the Acute Dialysis Quality Initiative (ADQI) created a new diagnostic classification that pulled together terms such as acute renal dysfunction and acute renal failure into one category, i.e., acute kidney injury (AKI). Based on changes in renal function that occurred over one week, three categories of AKI (Risk, Injury, and Failure) were defined by a) an incremental increase or threshold serum creatinine (SCr) or a decrease in the estimated glomerular filtration rate (eGFR) calculated from SCr; and b) the degree and duration of oliguria. Two additional categories (Loss and End-stage renal disease) were defined by the presence of established AKI for four weeks or three months, respectively. Subsequently, the ADQI coalesced into the Acute Kidney Injury Network (AKIN), and in 2007, published a new, simpler, three-stage definition known as the AKIN criteria (Table 1). These criteria are defined over 48 hr, making them more pertinent to the rapid diagnosis of AKI. The eGFR was discarded in favour of SCr alone, and definitions related to the duration of established AKI were dropped completely. Based on more recent evidence of the adverse impact on outcome of even slight increases in SCr, Stage 1 AKI is defined by as little as a 26.4 lmol L (0.3 mg dL) increase in SCr. The oliguria criteria in AKIN remain virtually unchanged from the RIFLE criteria, except for shorter and stricter thresholds in Stage 3. Subsequently, the AKIN criteria have achieved widespread acceptance in clinical medicine as well as outcomes research. In this issue of the Journal, Odutayo et al. publish the results of a prospective epidemiologic 30-day study on the impact of AKI in 603 patients admitted to intensive care units (ICUs) in five Canadian hospitals. This compelling study highlights the extraordinarily adverse effect that AKI has on patient outcomes. Using AKIN criteria, the authors found that a little more than a quarter of ICU patients developed AKI, which was associated with an almost fourfold increase in 30-day mortality (29.2% vs 8.6% in patients without AKI). About 60% of patients had Stage 1 AKI, with an equal distribution of Stage 2 and 3 AKI between the remaining 40%. Mortality increased significantly with the severity of AKI (Stage 1, 19.2%; Stage 2, 37.9%; Stage 3, 51.5%). The authors also evaluated the risk factors associated with AKI in the ICU population that they studied. Perhaps unsurprisingly, there were significant associations of AKI with increasing age, male sex, and cardiovascular and metabolic disease (hypertension, diabetes, coronary artery disease). Patients who developed AKI were more likely to be on mechanical ventilation or vasopressor therapy, with higher SCr, blood urea nitrogen (BUN), and white blood cell count, and lower pH, serum bicarbonate, and hemoglobin. Finally, the authors found that the BUN and serum bicarbonate values at the time of AKI diagnosis were significant predictors of mortality, with an area under the receiver-operating characteristic curve (AUC) of 0.79. However, Odutayo et al. themselves acknowledge that ‘‘...their ability to identify, with any reliability, those patients who will die is modest.’’ R. N. Sladen, MBChB (&) Division of Critical Care Medicine, Department of Anesthesiology, College of Physicians & Surgeons of Columbia University, PH 527-B, 630 West 168th St, New York, NY 10032, USA e-mail: rs543@columbia.edu