Medicaid Expansion’s Impact on Emergency Department Use by State and Payer

  • Fan Zhao
    Department of Epidemiology, Fielding School of Public Health, University of California, Los Angeles, Los Angeles, CA, USA
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  • Roch A. Nianogo
    Correspondence: Roch A. Nianogo, MD, PhD, Department of Epidemiology, Fielding School of Public Health, University of California, Los Angeles, 650 Charles E. Young Dr South, Los Angeles, CA 90095-1772, USA.
    Department of Epidemiology, Fielding School of Public Health, University of California, Los Angeles, Los Angeles, CA, USA

    California Center for Population Research, Los Angeles, CA, USA
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Published:October 25, 2021DOI:


      • Existing evidence on how the Medicaid Expansion Program affects inefficient and expensive emergency department (ED) visits is inconsistent.
      • The Medicaid expansion increased Medicaid’s share of ED visits and decreased private insurance’s share of ED visits. Uninsured share of ED visits decreased in most states, especially in the same states that experienced the largest increase of Medicaid enrollment.
      • States that experienced the largest increase (>60%) in Medicaid enrollment seem to experience an increase in total ED visits although such results did not reach statistical significance.



      The Affordable Care Act’s Medicaid Expansion Program influences healthcare use by increasing insurance coverage. Of particular interest is how this will affect inefficient and expensive emergency department (ED) visits. We estimated the impact of the Medicaid expansion on ED use by states and payer (Medicaid, private insurance, and uninsured) 5 years after the implementation of the Medicaid expansion and illustrated the use of the generalized synthetic control method.


      In this quasi-experiment study, we implemented the generalized synthetic control method to compare states with Medicaid expansion and states without Medicaid expansion. Data were from the Healthcare Cost Utilization Project Fast Stats, which cover >95% of all ED visits. We included states with complete data from 2010 to 2018.


      Overall, the Medicaid expansion increased Medicaid share of ED visits (average treatment effect on the treated [ATT] 11.39%; 95% confidence interval [CI] 8.76-14.02) and decreased private share of ED visits (ATT −5.80%; 95% CI −7.40 to −4.12) and uninsured share of ED visits (ATT −6.66%; 95% CI −9.78 to −3.55).


      Medicaid Expansion Program shifted ED payer mix to Medicaid ED visits from private insurance and uninsured ED visits for adults at age of 19 to 64 years, whereas its effect on total ED volume is mixed among states. States that experienced the largest increase in Medicaid enrollment seem to experience an increase in ED visits although such results did not reach statistical significance.


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