Geographical variation in physician supply and its relationship to utilization of care across older adults in the United States.

Introduction: Scholars express concern that general practitioner shortages and specialist surpluses induce overspecialization, with overuse of costly specialist services and underuse of cost-effective primary care services. Yet few studies directly assess the relationship between physician supply and patient utilization. Given this gap, this paper examines the associations between physician supply, care utilization, and patient need and whether patients use more specialists in areas with lower primary care supply. Methods: Using a 20% sample of 2018 Medicare fee-for-service claims, this paper first assessed the correlation between county physician densities and county physician visits. It then modeled individual patient consumption of primary and specialty care services in relation to physician supply through linear regression, adjusting for health and demographics. Results: While county supplies of primary care practitioners (PCPs) and specialists were positively correlated, we found no correlation between local PCP supply and local primary care visits. We also found no evidence that patients substitute specialist care for primary care, even in areas with PCP shortages. Conclusion: These findings suggest that factors other than PCP supply play an important role in primary care underuse. Scholars should also consider how care models, limited gatekeeping, and excess consumption among well-resourced populations influence the distribution of primary care utilization.