the fall in glomerular filtration rate and the rise in serum creatinine, which delays the diagnosis of AKI. A key goal in AKI management is early detection and intervention. The hourly measurement of urine volume provides the opportunity to treat urine flow as a continuous physiological variable, providing more time points for early detection of AKI than serum creatinine assessed once or twice a day. Urine output changes raise red flags to the physicians for an ongoing injury to the kidneys possibly leading to an early intervention. In a recent review, Goldstein and Chawla proposed the concept of ‘renal angina’ to trigger additional attention to the kidney . Oliguria is one of the three objective criteria along with any increase in serum creatinine and fluid overload that should prompt the concern of evolving AKI. Despite Dr Schiffl’s concerns we are confident that our findings will not “muddy the waters” for AKI. Rather, we have proposed a new methodology to standardize the urine output criteria in situations where an hourly urine flow measurement may not be available. Although the clinical relevance of the hourly assessment of the urine flow is not questioned, the 6-h block interval may be the only available information in some scenarios. This parameter is also relevant in retrospective studies, when often the hourly information is not available. We believe our paper has raised some awareness about the importance of timely diagnosis of AKI and the use of urine output as an early biomarker. Furthermore, our data provide support that urine output is an important biomarker of renal function, associated with major outcomes. Despite the limitations in the use of urine output as an early biomarker, its changes provide a sensitive and easy mean to identify patients with early AKI. We acknowledge that there is a requirement of additional large multicentre trials capturing refined data on fluid balance, diuretics and other factors affecting the urine flow, to explore further the use of urine output as a biomarker.
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