Design, methods and populations of the CURV-24 study: a global systematic review and meta-analysis of 24-h urine volume and creatinine excretion in community-dwelling adults.

Introduction: Urinary biomarker concentrations are increasingly used to assess intake, exposure or excretion, often by applying pre-specified 24-h urine volume values or normalization to creatinine based on prediction-derived estimates. However, both urine volume and creatinine excretion rates can vary substantially by sex, BMI, protein and salt intake, ethnicity, food culture, and short- and long-term climate. Ignoring such population-level differences may bias temporal, spatial, and cross population comparisons and undermine the validity of fixed reference ranges. CURV-24 is a global systematic review and meta-analysis designed to investigate this variability and its determinants in community-dwelling populations. The aim of the current article is to present the design and methods of CURV-24. We further describe the included studies and populations. Taken together, readers should be positioned to assess the quality and representativeness of the study data in the forthcoming meta-analyses. Methods: Using a broad, piloted syntax, we searched multiple bibliographic databases and registries (Medline, Embase, Web of Science, WHO Global Index Medicus, EBSCO Global Health, Cochrane, PROSPERO) without lan guage restrictions. Records published between 2015 and 2024 were investigated. We included primary studies with at least one timed 24-h urine collection from non-manipulated everyday conditions. Extensive efforts were made to identify companion reports and complementary datasets from authors and repositories. A tailored risk-of-bias and certainty-of-evidence framework, adapted from Grading of Recommendations Assessment, Development and Evaluation (GRADE), was developed to address population, collection, and analytical biases. The Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) and Meta-analysis Of Observational Studies in Epidemiology (MOOSE) have guided the development and reporting plans of the project. Results: The final dataset comprises 212 studies from 71 countries and 234 national populations, including approximately 270,000 individuals, of whom three-quarters were studied after 2010. About half of the studies examined more than 200 participants and one third used census-based probability sampling. Complete subpopulation data by sex, age, location, and ethnicity are available from a substantial subset of studies. Discussion: CURV-24 provides the most extensive synthesis to date of 24-h urine volume and creatinine excretion in populations worldwide. The project is positioned to improve hydration adjustment of urinary biomarkers, challenge fixed "one-size fits-all" assumptions and inform future best-practice guidelines in epidemiologi cal biomarker research. Looking ahead, CURV-24 has the potential to generate hypotheses on the relationship between climate and human hydration. Systematic review registration: https://www.crd.york.ac.uk/PROSPERO/view/CRD42024495900.