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Background: The urea creatinine ratio (UCR) is important in the clinical assessment of several medical conditions, including acute kidney injury and gastrointestinal bleeding. However, accurate and robust paediatric reference intervals (RIs) for this ratio have not been well established. Here, we determined age- and sex-specific discrete and continuous RIs for UCR in the Canadian Laboratory Initiative on Paediatric Reference Intervals (CALIPER) cohort of healthy children and adolescents for the first time., Methods: UCR was calculated for approximately 1030 CALIPER participants using retrospective urea and creatinine (both Jaffe and enzymatic methods) normative data. Partitions were determined using the Harris & Boyd statistical method. Discrete RIs were established in accordance with the Clinical and Laboratory Standards Institute (CLSI) guidelines. Continuous RIs were established using nonparametric quantile regression., Results: Several age- and sex-specific partitions were necessary to capture dynamic physiological trends associated with this ratio throughout childhood and adolescence, highlighting the benefit of continuous RI establishment. Established UCR RIs also demonstrated marked differences between Jaffe and enzymatic assay methods., Conclusion: Our results clearly demonstrate the critical need for RI stratification by important covariates such as age, sex, and creatinine assay methodology for paediatric UCR test result interpretation. These data contribute to our understanding of normative UCR values in childhood and adolescence and can be expected to improve paediatric test result interpretation in clinical laboratories that report this ratio., (Copyright © 2019 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.)