Introduction:
The remnant cholesterol to high-density lipoprotein cholesterol ratio (RC/HDL-C) reflects atherogenic lipid burden. We investigated its association with mortality in patients with chronic obstructive pulmonary disease (COPD) and the mediating role of body mass index (BMI).

Material and methods:
This cross-sectional study included 1,307 COPD patients from NHANES 2007–2018. Multivariable Cox proportional hazards models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs). Restricted cubic splines (RCS) were employed to assess dose-response relationships. Mediation analysis was conducted to quantify the extent to which BMI mediated the association between RC/HDL-C and mortality.

Results:
During a median follow-up of 104 months, 226 all-cause and 74 cardiovascular deaths occurred. After adjusting for potential confounders, elevated RC/HDL-C was independently associated with higher risks of all-cause (HR = 2.16, 95% CI: 1.64–2.86) and cardiovascular mortality (HR = 3.30, 95% CI: 2.19–4.99). Mediation analysis indicated that BMI exerted a suppression effect on the association with all-cause mortality, masking 11.0% of the RC/HDL-C-associated risk. However, no significant mediating effect of BMI was observed for cardiovascular mortality. Stratified analyses indicated that these associations were more pronounced in patients younger than 65 years.

Conclusions:
RC/HDL-C is independently associated with all-cause and cardiovascular mortality in COPD patients. While higher BMI partially suppresses the association between RC/HDL-C and all-cause mortality, no mediating effect is observed for cardiovascular mortality. The associations were more pronounced in younger patients, suggesting potential value for risk stratification in this subgroup.