Spatial Accessibility and Health-System Concentration of Reachable Hospital Capacity in New Jersey

Background: Spatial accessibility measures estimate how much hospital capacity a community can reach but do not show how that capacity is distributed across health systems. A community may have several reachable hospitals but still depend on one or a few systems. Methods: We used the enhanced two-step floating catchment area method to estimate staffed-bed spatial accessibility for New Jersey census tracts. We included short-term acute care hospitals in New Jersey and neighboring states within a 30-minute drive of at least one New Jersey tract. For each tract, we grouped hospital-level accessibility contributions by health system and calculated a Herfindahl-Hirschman Index (HHI). Spearman correlations examined associations of E2SFCA and HHI with tract-level sociodemographic characteristics. Results: Of 163 reachable hospitals, 94 were outside New Jersey and supplied 62.7% of reachable staffed beds. Across 2,172 tracts, the mean E2SFCA score was 1.98 beds per 1,000 distance-weighted residents; among 2,161 tracts with positive scores, the median HHI was 0.31. Using cutoffs of E2SFCA <2.0 and HHI [&ge;]0.18, 934 tracts met both criteria, while an E2SFCA-only rule missed 655 tracts with higher accessibility but high concentration. E2SFCA and HHI correlations had opposite signs for seven of eight sociodemographic characteristics. Conclusions: E2SFCA and HHI classified New Jersey tracts differently under the illustrative cutoffs. Contribution-based HHI identifies tracts in which a closure, service relocation, or capacity reduction involving a major contributing system could affect a large share of modeled accessible capacity. Keywords: Spatial accessibility; hospitals; E2SFCA; health systems; Herfindahl-Hirschman Index