Lower ambulatory care availability and greater hospital capacity are associated with higher hospital case volumes.

Introduction: The German hospital reform introduces population-based planning to allocate hospital budgets, considering each hospital's role in meeting regional care needs. However, current hospital case numbers may reflect supply-side factors, such as physician density and socioeconomic disparities, rather than actual morbidity. Ambulatory care utilization inversely correlates with hospital usage, emphasizing the need to integrate ambulatory sector data into hospital planning. This study examines factors influencing hospital and office-based case numbers at the district level.

Methods: Linking 2021 data from the Federal and State Statistical Offices, INKAR data and health insurance claims data in Germany at the district level, a multiple linear regression model assessed the association between case counts in hospitals or office-based practices per 10,000 residents and distance to the nearest general practitioner (GP), as well as hospital bed and GP density. The Global Moran's I as well as a geographically weighted regression (GWR) analysis were conducted to assess regional differences.

Results: Multiple linear regression revealed that greater GP distance, fewer GPs and more hospital beds were linked to more hospital cases, while office-based cases rose with shorter GP distance. Global Moran's I confirmed spatial clustering, and GWR revealed heterogeneous effects of primary-care access on hospital admissions, whereas bed capacity uniformly increased hospital cases and shorter GP distances consistently predicted more office visits across Germany.

Discussion: Our findings align with research showing supply-induced demand of hospital cases and emphasize the need for coordinated hospital and ambulatory care planning to improve access, reduce unnecessary hospital admissions, and optimize patient outcomes.