Exploring the inverse relationship between serum total bilirubin and systemic immune-inflammation index: insights from NHANES data (2009–2018)

Abstract

Abstract Background Bilirubin is known for its multifaceted attributes, including antioxidant, anti-inflammatory, immunomodulatory, and antiapoptotic properties. The systemic immune-inflammation index (SII) is a recent marker that reflects the balance between inflammation and immune response. Despite the wealth of information available on bilirubin’s diverse functionalities, the potential correlation between the total bilirubin (TB) levels and SII has not been investigated so far. Methods Leveraging data from the National Health and Nutrition Examination Survey spanning 2009–2018, the TB levels were categorized using tertiles. Employing the chi-squared test with Rao and Scott’s second-order correction and Spearman’s rank correlation analysis, the association between TB and SII was examined. The potential nonlinearities between TB and SII were evaluated using restricted cubic spline (RCS) analysis. Weighted linear regression, adjusted for covariates, was used to explore the correlation between TB and SII, with further subgroup analyses. Results A total of 16,858 participants were included, and the findings revealed significant SII variations across TB tertiles (p < 0.001). The third tertile (Q3) exhibited the lowest SII level at 495.73 (295.00) 1000 cells/µL. Spearman rank correlation disclosed the negative association between TB and SII. RCS analysis exposed the lack of statistically significant variations in the nonlinear relationship (p > 0.05), thereby providing support for a linear relationship. Weighted linear regression analysis underscored the negative correlation between TB and SII (β 95% CI − 3.9 [− 5.0 to  − 2.9], p < 0.001). The increase in the TB levels is associated with a significant linear trend toward decreasing SII. After controlling for relative covariates, this negative correlation increased (p < 0.001). Subgroup analysis confirmed the significant negative TB–SII association. Conclusion A notable negative correlation between TB and SII implies the potential protective effects of bilirubin in inflammation-related diseases.