Cardiometabolic index and advanced cardiovascular–kidney–metabolic syndrome in two population-based cohorts from the USA and China

Background The cardiometabolic index (CMI) integrates central adiposity with atherogenic dyslipidaemia and may identify individuals at elevated cardiometabolic risk. However, evidence linking CMI to advanced cardiovascular–kidney–metabolic (CKM) syndrome across diverse populations remains limited.Methods We conducted cross-sectional analyses on adults aged ≥ 45 years using data from NHANES (2001–2018) and CHARLS (2011/2015). CMI was calculated as (triglycerides/HDL-cholesterol) × (waist/height). The primary outcome was advanced CKM (stages 3–4). Multivariable logistic regression estimated odds ratios (ORs) and 95% confidence intervals (CIs). Restricted cubic splines and two-piecewise regression assessed non-linearity and thresholds.Results The analytic sample included 10,198 participants from NHANES and 17,285 from CHARLS. Higher CMI was independently associated with advanced CKM (NHANES: OR 1.35, 95% CI 1.15–1.57; CHARLS: OR 1.86, 95% CI 1.70–2.04). In quartile analyses, the highest CMI quartile demonstrated the strongest associations compared to the lowest (NHANES: OR 1.72, 95% CI 1.30–2.26; CHARLS: OR 3.33, 95% CI 2.85–3.89). While the relationship was linear in NHANES, it was non-linear in CHARLS (P < 0.001), with a threshold effect at CMI = 0.509. Below this threshold, risk increased sharply (OR 3.879, 95% CI 2.137–7.040), whereas above it, the association was attenuated but remained significant (OR 1.528, 95% CI 1.360–1.717).Conclusions CMI was associated with an increased risk of advanced CKM in two large, nationally representative cohorts. These findings support the potential utility of CMI in risk stratification. Future prospective studies are needed to confirm its predictive value and define clinically relevant thresholds.