Remnant Cholesterol Inflammatory Index and Its Association With All‐Cause Mortality Among General Population and Individuals With Cardiovascular–Kidney–Metabolic Syndrome Stages 0–3: Evidence From Two Nationwide Studies

BACKGROUND: Early identification of mortality risk in cardiovascular-kidney-metabolic (CKM) stages 0-3 remains challenging. The remnant cholesterol inflammatory index (RCII) has been associated with adverse outcomes, but its comparative prognostic performance in early CKM stages is unclear. METHODS: We analyzed data from two nationally representative cohorts, the U.S. National Health and Nutrition Examination Survey (NHANES) and the China Health and Retirement Longitudinal Study (CHARLS). RCII was compared with established metabolic indices for all-cause mortality using Cox regression, receiver operating characteristic analyses, and restricted cubic splines, with stratification by CKM stages. RESULTS: Across both cohorts, higher RCII was consistently associated with increased all-cause mortality in the general population and among individuals with CKM stages 0-3. RCII demonstrated superior discriminatory performance compared with other metabolic indices. These associations remained robust after multivariable adjustment and were driven primarily by individuals in the highest RCII category. In NHANES, RCII showed comparable associations with cardiovascular and non-cardiovascular mortality. CONCLUSION: RCII is a strong and independent predictor of all-cause mortality across diverse populations and CKM stages 0-3.