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Background: The baseline creatinine level is central in the Kidney Disease Improving Global Outcomes (KDIGO) criteria of AKI, but baseline creatinine is often inconsistently defined or unavailable in AKI research. We examined the rate, characteristics, and 30-day mortality of AKI in five AKI cohorts created using different definitions of baseline creatinine., Methods: This nationwide cohort study included all individuals aged ≥18 years in Denmark with a creatinine measurement in 2017. Applying the KDIGO criteria, we created four AKI cohorts using four different baseline definitions (most recent, mean, or median value of outpatient creatinine 365-368 days before, or median value 90-98 days before, if available, otherwise median value 365-391 days before) and one AKI cohort not using a baseline value. AKI rate and the distribution of age, sex, baseline creatinine, and comorbidity were described for each AKI cohort, and the 30-day all-cause mortality was estimated using the Kaplan-Meier method., Results: The study included 2,095,850 adults with at least one creatinine measurement in 2017. The four different baseline definitions identified between 61,189 and 62,597 AKI episodes. The AKI rate in these four cohorts was 13-14 per 1000 person-years, and 30-day all-cause mortality was 17%-18%. The cohort created without using a baseline creatinine included 37,659 AKI episodes, corresponding to an AKI rate of 8.2 per 1000 person-years and a 30-day mortality of 23%. All five cohorts were similar regarding age, sex, and comorbidity., Conclusions: In a population-based setting with available outpatient baseline creatinine, different baseline creatinine definitions revealed comparable AKI cohorts, whereas the lack of a baseline creatinine when defining AKI led to a smaller AKI cohort with a higher mortality. These findings underscore the importance of availability and consistent use of an outpatient baseline creatinine, particulary in studies of community-acquired AKI., Competing Interests: C. Christiansen reports that various companies provided research grants to Aarhus University. U. Heide-Jørgensen reports that the Department of Clinical Epidemiology at Aarhus University Hospital is involved in studies with funding from various companies as research grants to (and administered by) Aarhus University but that none of these studies are related to the current study. S. Vestergaard reports that the Department of Clinical Epidemiology is involved in studies with funding from various companies as research grants to (and administered by) Aarhus University. All remaining authors have nothing to disclose., (Copyright © 2022 by the American Society of Nephrology.)