Investigating the causal effect of socioeconomic status on quality of care under a universal health insurance system-a marginal structural model approach

Background: Social disparities in healthcare persist in the US despite the expansion of Medicaid under the Affordable Care Act. We investigated the causal impact of socioeconomic status on the quality of care in a setting with minimal confounding bias from race, insurance type, and access to care. Methods: We designed a retrospective population-based study with a random 25% sample of adult Taiwan population enrolled in Taiwan's National Health Insurance system from 2000 to 2016. Patient's income levels were categorized into low-income group (<25th percentile) and high-income group (≥25th percentile). We used marginal structural modeling analysis to calculate the odds of hospital admissions for 11 ambulatory care sensitive conditions identified by the Agency for Healthcare Research and Quality and the odds of having an Elixhauser comorbidity index greater than zero for low-income patients. Results: Among 2,844,334 patients, those in lower-income group had 1.28 greater odds (95% CI 1.24-1.33) of experiencing preventable hospitalizations, and 1.04 greater odds (95% CI 1.03-1.05) of having a comorbid condition in comparison to high-income group. Conclusions: Income was shown to be a causal factor in a patient's health and a determinant of the quality of care received even with equitable access to care under a universal health insurance system. Policies focusing on addressing income as an important upstream causal determinant of health to provide support to patients in lower socioeconomic status will be effective in improving health outcomes for this vulnerable social stratum.