Objective

To quantify impacts of early Affordable Care Act (ACA) Medicaid expansions on Medicaid participation for primary care physicians.

Data Sources

The study uses secondary Medicaid Analytic eXtract (MAX) data from the United States for 2009-2012 as well as secondary National Plan and Provider Enumeration System (NPPES) data from the United States for 2015.

Study Design

The study uses a quasi-experimental difference-in-differences study design where the policy change is Medicaid expansion in 6 states that adopted early ACA Medicaid expansions during 2010 and 2011: California, Connecticut, the District of Columbia, Minnesota, New Jersey, and Washington. The key outcome variables are five monthly measures of physician participation: the number of Medicaid visits, the number of Medicaid patients, seeing at least 1 Medicaid patient, seeing at least 25 Medicaid patients, and seeing at least 50 Medicaid patients.

Data Collection/Extraction Methods

The sample consists of all physicians who were active between 2005 and 2015 according to the NPPES.

Principal Findings

For primary care physicians, Medicaid expansion led to a 29% increase in Medicaid visits (5.88 per month; 95% CI: 2.49 to 9.27), a 29% increase in Medicaid patients (4.59 per month; 95% CI: 2.16 to 7.02), and did not affect the probability of any Medicaid participation. Medicaid expansion also led to a 22% increase in the probability of seeing at least 25 Medicaid patients per month (4.58 percentage points; 95% CI: 1.27 to 7.89) and a 31% increase in the probability of seeing at least 50 Medicaid patients per month (2.99 percentage points; 95% CI: 0.99 to 4.99).

Conclusions

Early ACA Medicaid expansions led to increased Medicaid visits for primary care physicians but did not affect the probability of any Medicaid participation. Primary care physicians who had previously served Medicaid patients responded to early ACA Medicaid expansions by serving substantially more Medicaid patients.