Nonlinear associations of C-reactive protein–triglyceride glucose index with osteoporosis and bone mineral density in the elderly: a cross-sectional study

The C-reactive protein–triglyceride glucose index (CTI) is a novel composite biomarker that integrates insulin resistance (IR) and inflammation. This study aimed to investigate the association of CTI with osteoporosis (OP) and bone mineral density (BMD) in the elderly, and to evaluate its potential utility in identifying individuals with higher odds of OP. This single-center cross-sectional study enrolled 3,482 patients aged ≥ 60 years at initial admission between January 1, 2021, and December 31, 2024. Participants were categorized into the OP group (BMD < 80 mg/cm³) and the non-OP group (BMD ≥ 80 mg/cm³) based on lumbar BMD measured by quantitative computed tomography (QCT). Multivariable logistic regression was used to assess the association between CTI and OP, and restricted cubic splines (RCS) were applied to explore nonlinear relationships of CTI with OP and BMD. Subgroup analyses and sensitivity analyses (excluding individuals with CRP > 10 mg/L) were performed to evaluate the robustness of the findings. CTI levels were significantly higher in the OP group than in the non-OP group (8.45 vs. 8.26, P < 0.001). Multivariable logistic regression showed that each 1-unit increase in CTI was associated with 60% higher odds of OP (OR = 1.60, 95% CI: 1.41–1.80, P < 0.001). Quartile analysis revealed significantly elevated odds of OP in Q2 (OR = 1.47, 95% CI: 1.19–1.81, P < 0.001), Q3 (OR = 1.52, 95% CI: 1.23–1.88, P < 0.001), and Q4 (OR = 2.14, 95% CI: 1.70–2.69, P < 0.001) compared with Q1, indicating a dose–response relationship (P for trend < 0.001). RCS analysis revealed a nonlinear association between CTI and OP (P for non-linearity = 0.004), with a significant increase in the odds of OP when CTI exceeded 8.35. A nonlinear relationship was also observed between CTI and BMD (P for non-linearity < 0.001), with a threshold effect. When CTI ≤ 7.54, each 1-unit increase in CTI was associated with a marked decrease in BMD (β = -26.54, 95% CI: -36.16 to -16.92, P < 0.001); above this threshold, the decline became more gradual (β = -3.44, 95% CI: -5.11 to -1.77, P < 0.001). Sensitivity analyses confirmed the robustness of the positive association between CTI and OP, and subgroup analyses indicated that the association was more pronounced in females and in individuals without diabetes. CTI exhibits nonlinear associations with OP and BMD in the elderly, with significant threshold effects. These findings suggest that CTI may be useful for identifying individuals with higher odds of QCT-defined lumbar osteoporosis. Not applicable.