Objectives To investigate whether health insurance generated improvements in cardiovascular risk factors (blood pressure and hemoglobin A(1c) (HbA(1c)) levels) for identifiable subpopulations, and using machine learning to identify characteristics of people predicted to benefit highly. Design Secondary analysis of randomized controlled trial. Setting Medicaid insurance coverage in 2008 for adults on low incomes (defined as lower than the federal-defined poverty line) in Oregon who were uninsured. Participants 12 134 participants from the Oregon Health Insurance Experiment with in-person data for health outcomes for both treatment and control groups. Interventions Health insurance (Medicaid) coverage. Main outcomes measures The conditional local average treatment effects of Medicaid coverage on systolic blood pressure and HbA(1c) using a machine learning causal forest algorithm (with instrumental variables). Characteristics of individuals with positive predicted benefits of Medicaid coverage based on the algorithm were compared with the characteristics of others. The effect of Medicaid coverage was calculated on blood pressure and HbA(1c) among individuals with high predicted benefits. Results In the in-person interview survey, mean systolic blood pressure was 119 (standard deviation 17) mm Hg and mean HbA(1c) concentrations was 5.3% (standard deviation 0.6%). Our causal forest model showed heterogeneity in the effect of Medicaid coverage on systolic blood pressure and HbA(1c). Individuals with lower baseline healthcare charges, for example, had higher predicted benefits from gaining Medicaid coverage. Medicaid coverage significantly lowered systolic blood pressure (-4.96 mm Hg (95% confidence interval -7.80 to -2.48)) for people predicted to benefit highly. HbA(1c) was also significantly reduced by Medicaid coverage for people with high predicted benefits, but the size was not clinically meaningful (-0.12% (-0.25% to -0.01%)). Conclusions Although Medicaid coverage did not improve cardiovascular risk factors on average, substantial heterogeneity was noted in the effects within that population. Individuals with high predicted benefits were more likely to have no or low prior healthcare charges, for example. Our findings suggest that Medicaid coverage leads to improved cardiovascular risk factors for some, particularly for blood pressure, although those benefits may be diluted by individuals who did not experience benefits.
