Giving urine biochemistry a second chance in acute kidney injury monitoring

Most studies assessing urine biochemistry for acute kidney injury (AKI) monitoring rely on paradigms from the 1970s. It was proposed that a single measurement of urinary parameters in the presence of increased serum creatinine (sCr) could help understand AKI pathophysiology and predict its duration. However, those studies produced variable and controversial results. Recently, an alternative "urine biochemical approach" has been proposed. In contrast with the traditional approach, it includes sequential urine electrolyte assessment, evaluation before AKI diagnosis, and interpretation of avid sodium retention as a marker of renal microcirculatory stress instead of low renal perfusion. This review highlights the rationale of this alternative approach, which is focused on early urinary biochemical changes that precede increases in sCr as well as signs of renal recovery before decreases in sCr. The relevance of urine composition in conjunction with urine volume for a proper evaluation of renal function is emphasized. This new approach aims to enhance the utility of urinary biochemical parameters in AKI monitoring, particularly in patients who are critically ill.