Geographic disparities in accessibility to clinical studies for autoimmune skin disorders in the US: a retrospective review (2005–2024)

Abstract Equitable access to clinical studies is essential to ensuring that research findings are generalizable, inclusive, and reflective of all populations. When specific demographic or geographic groups are excluded from participation, the external validity of clinical research is undermined and inequities in treatment access are perpetuated. Autoimmune skin disorders are uncommon and often affect patients living far from major research centers, yet no comprehensive analysis of national clinical study accessibility for these conditions has been conducted. We conducted a retrospective, population-level analysis of study accessibility for autoimmune skin disorders in the United States from 2005 to 2024 using ClinicalTrials.gov and U.S. Census data. Population centers were defined as geographic centroids of ZIP codes, and the distance from each population center to the nearest study site was calculated. Trend analyses assessed changes in accessibility over time and differences across racial and urban–rural categories. Although the number of studies has increased over time (approximately six additional studies annually), travel distances to the nearest study site remain substantial. The average distance has decreased modestly by about 3.4 km per year, but 40% of Americans still live more than 200 km from the closest study site, with the greatest distances observed among rural and American Indian populations. Expanding population coverage requires rapidly increasing travel distances. These findings highlight persistent disparities in access to clinical studies. While decentralization and virtual study models may help reduce these gaps, further research is needed to determine whether such strategies effectively improve participation and representation.