A Comment on the Diagnosis and Definition of Acute Kidney Injury.

International criteria for describing the presence and severity of acute kidney injury (AKI) based on changes in serum creatinine concentration and/or degree of oliguria are now widely accepted. Subject of Review: Three recent articles have debated the definition and diagnosis of AKI, offering conflicting opinions. On one side [Lancet 2017; 389: 779-781 and Nephrology Times 2018] an argument is made that a focus on creatinine-based staging has de-emphasised the traditional clinical approach of determining cause of AKI (pre-renal, renal or post-renal), and that any classification system based on serum creatinine is inherently flawed. The opposing argument, is that serum creatinine-based staging brings value via the consistent, robust and gradated associations between AKI stage and outcomes, and that many cases of AKI have multiple co-existing intra- and extra-renal processes that do not fit neatly into the traditional aetiological groupings [Lancet 2018; 391: 202-203]. Second Opinion: Determining the cause of AKI is a key element of clinical management, so it is important that AKI is not regarded as a single disease, rather a syndrome with multiple potential causes. This article critiques current, clinical approaches to determining AKI aetiology alongside future areas in which significant developments in patient phenotyping based on pathophysiological principles may occur. In the absence of current alternatives to serum creatinine, current AKI criteria (e.g., those from Kidney Disease Improving Global Outcomes) bring significant advantages in clinical and research environments, including facilitation of efforts to address current variations in the delivery of AKI care. However, their application needs to be accompanied by 2 aspects: an appreciation of the limitations of serum creatinine as a diagnostic test; and an absolute requirement for clinical assessment and diagnostic workup to establish the cause of AKI.