The consequences of firm scope and scale on patient access to healthcare.

Objective: The aim was to quantify changes in the market structure of primary care physicians and examine its relationship with access to care.

Data Sources And Study Setting: We created measures of market structure from a 5% sample of Medicare fee-for-service claims and examined access to care using nationally representative data from the Medical Expenditure Panel Survey (MEPS). Our study spanned from 2008 to 2019.

Study Design: We used a linear probability model to estimate the relationship between access to care and two measures of market structure: concentration, measured by the Herfindahl-Hirschman Index (HHI), and vertical integration, measured by the market share of multispecialty firms. Our model controlled for year and ZIP code fixed effects, respondents' demographics and health status, and other measures of market structure.

Data Collection/extraction Methods: All adult respondents in the MEPS were included.

Principal Findings: The percentage of people living in concentrated ZIP codes (HHI above 1500) increased from 37% in 2008 to 53% in 2019. During the same period, the median market share of multispecialty firms rose from 30% to 48%. Respondents in highly concentrated ZIP codes (HHI over 2500) were 5.9 percentage points (95% CI: -1.4 to -10.4) less likely to report having access to immediate care than respondents in unconcentrated ZIP codes. The association was largest among Medicaid beneficiaries, a 17.3 percentage point reduction (95% CI: -5.1 to -29.4). When we applied a model that was robust to biases from treatments with staggered timing, the estimated association remained negative but was not statistically significant. We found no association between HHI and indicators for having a usual source of care and annual checkups. The multispecialty market share was negatively associated with checkups, but not other measures of access.

Conclusions: Increases in concentration may reduce some types of access to healthcare. These effects appear most pronounced among Medicaid beneficiaries.