Apolipoprotein B (APO.B) and All-Cause Mortality in Elderly Patients with Established Cardiovascular Disease: A Nonlinear Association and Threshold Effect Analysis Based on NHANES 2005–2016

Background: Cardiovascular disease (CVD) is a leading cause of death in people over 60 years old. Reliable prognostic markers for risk stratification in these elderly patients are urgently needed. Apolipoprotein B (APO.B) reflects atherogenic lipoprotein particle number, but its association with all-cause mortality in elderly CVD patients, especially nonlinear patterns and threshold effects, is unclear. Methods: This cohort study used data from the National Health and Nutrition Examination Survey (NHANES) 2005–2016, including 1206 elderly patients with established CVD aged over 60 years. APO.B was the exposure, stratified into three tertiles, and all-cause mortality was the primary outcome. Multivariate Cox regression, piece-wise linear regression, and subgroup analyses were conducted. Results: During follow-up, the middle APO.B tertile (75.00–97.00 mg/dL) was associated with lower all-cause mortality compared to the low tertile (adjusted hazard ratio [HR]=0.75, 95% confidence interval [CI]: 0.61–0.92, P=0.0068). A nonlinear relationship with a threshold at 83 mg/dL was found: below it, each 1 mg/dL increase in APO.B reduced mortality (odds ratio [OR]=0.98, 95% CI: 0.97–0.99, P<0.0001); above it, no association (OR=1.00, 95% CI: 1.00–1.01, P=0.1877). Protective effects were more pronounced in males, non-Hispanic Whites, former/current smokers, and those with physical activity <600 MET/week. Conclusions: APO.B was nonlinearly associated with all-cause mortality in elderly patients with cardiovascular disease. This finding adds to current knowledge on lipid-related risk assessment and offers a direction for subsequent research on cardiovascular outcomes in the elderly.