Long-term changes in employment status as a predictor of catastrophic health expenditure in South Korea

South Korea faces persistent challenges in healthcare financing, with high out-of-pocket payments driving catastrophic health expenditure (CHE) among vulnerable households. Employment instability is a critical yet understudied determinant of CHE. Prior studies have largely examined employment changes in binary terms over short time frames. This study investigated the longitudinal association between employment status trajectories (EST) and CHE among Korean adults. This study utilized data from the 9th to 17th waves of the National Survey of Tax and Benefit. Of the 7,809 participants enrolled in the 9th wave, 1,123 were excluded due to missing data on employment status or covariates, yielding a final analytical sample of 6,686 participants. The study period was divided into two sequential analytic phases. In the first phase, group-based trajectory modeling was applied to waves 9–12 to identify distinct EST groups separately for adults aged under 50 and those aged 50 and above. In the second phase, generalized estimating equations were applied to waves 12–17 to estimate the association between the identified EST groups and CHE occurrence; the 12th wave served as both the endpoint for trajectory identification and the baseline for this follow-up. This two-phase sequential design minimized potential reverse causality by establishing EST patterns independently, prior to CHE occurrence, thereby preserving temporal order. CHE was defined following World Health Organization criteria as out-of-pocket medical expenditure exceeding household capacity to pay, with sensitivity analyses conducted at 20%, 30%, and 40% thresholds. Among adults under 50 years, no significant association was observed between EST and CHE. Among those aged 50 and above, however, the sustained unemployment group showed 1.26, 1.25, and 1.34 times higher odds of CHE at the 20%, 30%, and 40% thresholds, respectively, compared to the sustained standard white-collar group. The “changed white-collar to unemployment” group similarly exhibited 1.32, 1.32, and 1.35 times higher odds at each threshold. Employment instability, particularly sustained unemployment and transitions into unemployment, significantly increases the risk of CHE among adults aged 50 and older in South Korea. Targeted medical cost support policies addressing groups with sustained unemployment or frequent employment transitions are essential to reducing household financial burdens from healthcare expenditure.