• Mara A G Hollander, Evan S Cole, Lindsay M Sabik, Jeremy M Kahn, ChangChung-Chou HCHDepartment of Biostatistics, University of Pittsburgh Graduate School of Public Health, Pittsburgh, PA; Division of General Internal Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA., Marian P Jarlenski, and Julie M Donohue.
• Johns Hopkins Bloomberg School of Public Health, Department of Health Policy and Management, Baltimore, MD. Electronic address: mholla23@jhu.edu.
• Ann Emerg Med. 2021 Jul 1; 78 (1): 57-67.

Study ObjectiveMore than 17 million people have gained health insurance coverage through the Patient Protection and Affordable Care Act's Medicaid expansion. Few studies have examined heterogeneity within the Medicaid expansion population. We do so based on time-varying patterns of emergency department (ED) and ambulatory care use, and characterize diagnoses associated with ED and ambulatory care visits to evaluate whether certain diagnoses predominate in individual trajectories.MethodWe used group-based multitrajectory modeling to jointly estimate trajectories of ambulatory care and ED utilization in the first 12 months of enrollment among Pennsylvania Medicaid expansion enrollees from 2015 to 2017.ResultsAmong 601,877 expansion enrollees, we identified 6 distinct groups based on joint trajectories of ED and ambulatory care use. Mean ED use varied across groups from 3.4 to 48.7 visits per 100 enrollees in the first month and between 2.8 and 44.0 visits per 100 enrollees in month 12. Mean ambulatory visit rates varied from 0.0 to 179 visits per 100 enrollees in the first month and from 0.0 to 274 visits in month 12. Rates of ED visits did not change over time, but rates of ambulatory care visits increased by at least 50% among 4 groups during the study period. Groups varied on chronic condition diagnoses, including mental health and substance use disorders, as well as diagnoses associated with ambulatory care visits.ConclusionWe found substantial variation in rates of ED and ambulatory care use across empirically defined subgroups of Medicaid expansion enrollees. We also identified heterogeneity among the diagnoses associated with these visits. This data-driven approach may be used to target resources to encourage efficient use of ED services and support engagement with ambulatory care clinicians.Copyright © 2021 American College of Emergency Physicians. Published by Elsevier Inc. All rights reserved.

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