Income-Based Inequalities in Health System Performance in the US and South Korea.

Importance: Income is a key social determinant of health, yet its influence on health system performance may differ across settings. Cross-national comparisons can help identify where income-related disparities are most pronounced and inform targeted policy responses; the US and South Korea are 2 members of the Organisation for Economic Co-operation and Development with high poverty rates but different health systems. Objective: To compare health system performance and income-related inequalities in health system performance between the US and South Korea. Design, Setting, and Participants: This repeated cross-sectional study including nationally representative samples of noninstitutionalized adults from the US and South Korea used data from the Medical Expenditure Panel Survey (MEPS; 2010-2019), National Health and Nutrition Examination Survey (NHANES; 2009-2018), Korean Health Panel Study (KHPS; 2010-2019), and Korean National Health and Nutrition Examination Survey (KNHANES; 2010-2019). Data were analyzed from March 2024 to March 2025. Exposures: Annual household income, categorized into country-specific deciles. Main Outcomes and Measures: The main outcomes were 30 indicators across 6 domains: health care spending, health care utilization, access to care, health status, behavioral risk factors, and clinical outcomes. To evaluate income-related inequalities in outcomes, adjusted mean values across income deciles were estimated using regression models. Results: The sample included 224 168 US adults (female: 51.1% in MEPS, 51.7% in NHANES) and 179 452 South Korean adults (female: 52.4% in KHPS, 56.1% in KNHANES). Mean (SD) age was 46.6 (18.0) years in MEPS, 46.5 (17.4) years in NHANES, 47.7 (16.2) years in KHPS, and 50.5 (17.1) years in KNHANES. US adults had higher mean total health care spending (lowest income decile: $7852 [95% CI, $7456-$8247]; highest decile: $6510 [95% CI, $6218-$6802]) than South Korean adults (lowest decile: $1184 [95% CI, $1105-$1263]; highest decile: $1025 [95% CI, $950-$1100]) despite similar levels of self-reported good health. A 1-decile increase in income was associated with a difference of -$142 (95% CI, -$179 to -$104) in total health care spending in the US compared with -$33 (95% CI, -$41 to -$25) in South Korea. A 1-decile increase in income was associated with an increase of 2.4 (95% CI, 2.3-2.5) percentage points (pp) in self-reported good health in the US compared with 1.5 (95% CI, 1.4-1.6) pp in South Korea. Income-related disparities in preventive service use were also larger in the US, ranging from 0.2 (95% CI, 0.2-0.2) pp for cervical cancer screening to 4.0 (95% CI, 3.9-4.1) pp for dental checkups. In South Korea, disparities ranged from 0.6 (95% CI, 0.4-0.8) pp for dental checkups to 2.0 (1.8-2.2) pp for routine checkups. Similar income gradients were observed in access to care and behavioral risk factors. Differences in clinical outcomes were modest in both countries. Conclusions and Relevance: In this cross-sectional study, income was associated with disparities in health system performance in both the US and South Korea, with larger differences by income in the US. The findings suggest that structural and systemic policy efforts are needed to address income-based health inequalities, particularly in the US.