Best Practices in Demographic Data Collection for Equity, Diversity, and Inclusion in Rare Disease Research: A Systematic Review.

PURPOSE: Rare diseases affect small, dispersed populations and are often studied through multisite designs where equity-relevant demographic data are essential for inclusive recruitment and accurate interpretation. This study examined how sociodemographic variables are collected and reported in rare disease research and evaluated their alignment with the PROGRESS-Plus framework, which outlines Place of residence, Race/ethnicity/culture/language, Occupation, Gender/sex, Religion, Education, Socioeconomic status, social capital, and additional "Plus" factors such as age and disability status. METHODS: A systematic review of peer-reviewed articles was conducted alongside an environmental scan of demographic instruments from governmental, health-system, academic, and rare disease organizations. Screening and extraction coded variables as reported, indirectly derivable, or not reported and compared them with established standards. RESULTS: Of 647 records identified, 37 met inclusion criteria. Reporting was dominated by age and sex, while most other equity-relevant variables including gender identity, sexual orientation, race/ethnicity, distinctions-based Indigenous identity, socioeconomic position, language, migration, disability/function, religion, occupation, and social capital, were inconsistently captured. Environmental scan instruments were more comprehensive, revealing a capture-to-reporting gap. CONCLUSION: Demographic reporting in rare disease research is heterogeneous and insufficient for equity-focused analyses. A concise, standards-aligned sociodemographic dataset is needed to improve transparency, comparability, and detection of inequities across rare disease populations.