How Much Chronic Disease Out-of-Pocket Expenditure Runs Through Pain? A Prospective Counterfactual Decomposition of Five-Year Korean Panel Data.

Background and Objectives: Pain has been proposed as a pathway linking chronic disease to healthcare spending, but existing estimates measure exposure, pain and cost in the same period and combine coefficients on incompatible scales. We asked how much of the disease-expenditure association runs through pain when exposure, pain and cost are separated in time. Materials and Methods: In the Korea Health Panel Survey (2019-2023; 54,845 adult person-years), chronic disease at year t, EQ-5D pain/discomfort at t + 1 and out-of-pocket payments during t + 2 were linked, conditioning on pain and payments at t. Direct and indirect effects were estimated by parametric g-computation (randomised-interventional analogues, two-part outcome model, exposure-specific adjustment sets, survey and censoring weights, false discovery rate control). Results: Among 25,692 triplets, pain/discomfort predicted the amount spent among healthcare users (cost ratio, CR 1.099, 95% confidence interval, CI 1.023-1.180) but showed no detectable association with whether care was used (odds ratio, OR 1.136, 0.918-1.406); the contemporaneous cost ratio was substantially larger (1.296), inflated by simultaneity. Three musculoskeletal conditions prospectively predicted pain (false discovery rate q < 0.001). Six conditions had indirect-effect intervals excluding zero, but none survived false discovery rate correction (minimum q = 0.19); where estimable, the proportion mediated was small (2.8% to 4.8%). Conclusions: The disease → pain and pain → payment associations are each established, but the disease-specific mediated amount is not confirmed after multiplicity correction. Contemporaneous designs overstate the indirect association; claims that pain causes a specified share of chronic disease expenditure are not supported.