Importance: The Affordable Care Act expanded access to Medicaid coverage in 2014 for individuals living in participating states. Whether expanded coverage was associated with increases in the use of outpatient surgical care, particularly among underserved populations, remains unknown. Objective: To evaluate the association between state participation in the Affordable Care Act Medicaid expansion reform and the use of outpatient surgical care. Design, Setting, and Participants: This case-control study used a quasi-experimental difference-in-differences design to compare the use of outpatient surgical care at the facility and state levels by patient demographic characteristics and payer categories (Medicaid, private insurance, and no insurance). Data from 2013 (before Medicaid expansion reform) and 2015 (after Medicaid expansion reform) were obtained from the State Ambulatory Surgery and Services Database of the Healthcare Cost and Utilization Project. The absolute and mean numbers of procedures performed at outpatient surgical centers in 2 states (Michigan and New York) that participated in Medicaid expansion (expansion states) were compared with those performed at outpatient surgical centers in 2 states (Florida and North Carolina) that did not participate in Medicaid expansion (nonexpansion states). The population-based sample included 207176 patients aged 18 to 64 years who received 4 common outpatient procedures (laparoscopic cholecystectomy, breast lumpectomy, open inguinal hernia repair, and laparoscopic inguinal hernia repair). Data were analyzed from May 19 to August 25, 2019. Interventions: State variation in the adoption of Medicaid expansion before and after expansion reform was implemented through the Affordable Care Act. Main Outcomes and Measures: Changes in the mean number of procedures performed at the facility level before and after Medicaid expansion reform in states with and without expanded Medicaid coverage. Results: A total of 207176 patients (106395 women [51.35%] and 100781 men [48.65%]; mean [SD] age, 45.7 [12.4] years) were included in the sample. Overall, 116752 procedures were performed in Medicaid expansion states and 90424 procedures in nonexpansion states. A 9.8% increase (95% CI, 0.4%-20.0%; P =.04) in cholecystectomies, a 26.1% increase (95% CI, 9.8%-44.7%; P =.001) in lumpectomies, and a 16.3% increase (95% CI, 2.9%-31.5%; P =.02) in laparoscopic inguinal hernia repairs were observed at the facility level in expansion states compared with nonexpansion states. Among patients with Medicaid coverage, the mean number of procedures performed in all 4 procedure categories increased between 60.5% (95% CI, 24.7%-106.6%; P <.001) and 79.2% (95% CI, 53.5%-109.2%; P <.001) at the facility level. The increases in the number of Medicaid patients who received treatment exceeded the reductions in the number of uninsured patients who received treatment with laparoscopic cholecystectomy, open inguinal hernia repair, and laparoscopic inguinal hernia repairs in expansion states compared with nonexpansion states. Black patients received more laparoscopic cholecystectomies, lumpectomies, and open inguinal hernia repairs in expansion states than in nonexpansion states. Conclusions and Relevance: Study results suggest that Medicaid expansion was associated with increases in the use of outpatient surgical care in states that participated in Medicaid expansion. Most of this increase represented patients who were newly treated rather than patients who converted from no insurance to Medicaid coverage.
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