Abstract
Socioeconomic status (SES) has been linked to mortality rates, with family income being a quantifiable marker of SES. However, the precise association between the family income-to-poverty ratio (PIR) and all-cause mortality in adults aged 40 and older remains unclear. A cross-sectional study was conducted using data from NHANES III, including 20,497 individuals. The PIR was used to assess financial status, and various demographic, lifestyle, and clinical factors were considered. Mortality data were collected from the NHANES III linked mortality file. The study revealed a non-linear association between PIR and all-cause mortality. The piecewise Cox proportional hazards regression model showed an inflection point at PIR 3.5. Below this threshold, the hazard ratio (HR) for all-cause mortality was 0.85 (95% CI 0.79-0.91), while above 3.5, the HR decreased to 0.66 (95% CI 0.57-0.76). Participants with lower income had a higher probability of all-cause mortality, with middle-income and high-income groups showing lower multivariate-adjusted HRs compared to the low-income group. This study provides evidence of a non-linear association between PIR and all-cause mortality in adults aged 40 and older, with an inflection point at PIR 3.5. These findings emphasize the importance of considering the non-linear relationship between family income and mortality when addressing socioeconomic health disparities.