Definition of hourly urine output influences reported incidence and staging of acute kidney injury

Acute kidney injury (AKI) is commonly defined using the KDIGO system, which includes criteria based on reduced urine output (UO). There is no consensus on whether UO should be measured using consecutive hourly readings or mean output. This makes KDIGO UO definition and staging of AKI vulnerable to inconsistency which has implications both for research and clinical practice. The objective of this study was to investigate whether the way in which UO is defined affects incidence and staging of AKI. We conducted a retrospective analysis of two single centre observational studies investigating (i) patients undergoing cardiac surgery and (ii) patients admitted to general intensive care units (ICU). AKI was identified using KDIGO serum creatinine (SCr) criteria and two methods of UO (UO : UO meeting KDIGO criteria in each consecutive hour; UO : mean hourly UO meeting KDIGO criteria). Data from 151 CICU and 150 ICU admissions were analysed. Incidence of AKI using SCr alone was 23.8% in CICU and 32% in ICU. Incidence increased in both groups when UO was considered, with inclusion of UO more than doubling reported incidence of AKI (CICU: UO 39.7%, UO 72.8%; ICU: UO 51.3%, UO 69.3%). In both groups UO led to a larger increase in KDIGO stage 1 but UO increased the incidence of KDIGO stage 2. We demonstrate a serious lack of clarity in the internationally accepted AKI definition leading to significant variability in reporting of AKI incidence.