Income-related inequality in out-of-pocket health-care expenditures under Taiwan's national health insurance system: An international comparable estimation based on A System of Health Accounts

National estimates of out-of-pocket health-care expenditures (OOP-HCEs) that use comparable international guidelines based on A Systems of Health Accounts (SHA) are generally unavailable in Taiwan. International comparable OOP are essential for designing universal health-coverage (UHC) policy. We designed an SHA-based household OOP questionnaire. A nationally representative cross-sectional survey was then conducted from January to August 2022. The final questionnaire was completed by 657 households and 1969 individuals. The total OOPs were divided into expenditures related to curative care (HC.1), rehabilitative care (HC.2), long-term care (HC.3), ancillary services (HC.4), and medical goods (HC.5). National estimates were calculated by accounting for the complex survey design. Variance was estimated through Taylor series linearization. The concentration index was calculated using household income as the ranking variable. We then identified factors contributing to the inequality in OOP distribution by household income. National estimates revealed an OOP of NT$424 billion, which accounted for 29.6% of Taiwan's national health expenditure in 2021. Private health insurance (PHI) reimbursements accounted for 9.0% of the total OOP. The OOPs for curative care and medical goods accounted for 50.1% and 39.0% of the total OOP, respectively. The OOPs after PHI reimbursements were progressive (concentration index = 0.103, P = 0.012). The frequency of medical-care use and the number of medical visits negatively affected progressive OOPs. International comparable OOPs revealed that under the Taiwanese National Health Insurance (NHI), OOPs can still be high. However, the NHI might have caused OOPs to be progressive from the perspective of income but regressive from the perspective of health status. Countries striving for UHC should consider the redistribution effect of public health insurance and possible inequalities in health.