Association of systemic inflammation indices with severe cognitive impairment in Chinese Han patients with Alzheimer’s disease: a cross-sectional study of NLR, PLR, SII, and SIRI

Objective The relationship between systemic inflammatory markers and cognitive impairment severity in Alzheimer’s disease (AD) remains unclear. This study aimed to investigate the association of the neutrophil-to-lymphocyte ratio (NLR) and additional composite inflammatory indices—platelet-to-lymphocyte ratio (PLR), systemic immune-inflammation index (SII), and systemic inflammation response index (SIRI)—with severe cognitive impairment (SCI) in Chinese Han patients with AD. Methods This cross-sectional study included 282 Chinese Han patients with AD. Sociodemographic characteristics and biochemical indicators were collected, and NLR, PLR, SII, and SIRI were calculated. Cognitive function was assessed using the Montreal Cognitive Assessment (MoCA), and SCI was defined as a MoCA score ≤9. Multivariable logistic regression was used to analyze the associations of the four inflammatory indicators with SCI risk. Smooth curve fitting and piecewise linear regression models were applied, with threshold effect analysis performed. Subgroup analyses were conducted according to sex, age, years of education, disease duration, diabetes, hemoglobin, triglycerides, and free triiodothyronine. Results After adjustment for covariates, NLR was linearly and positively associated with SCI risk (OR = 1.17, 95% CI: 1.00–1.37, p = 0.048). Neither PLR nor SIRI showed a significant association with SCI after multivariable adjustment ( p > 0.05). In contrast, SII exhibited a non-linear association with SCI risk, with an inflection point at approximately 861.9. Above this threshold, each unit increase in SII was associated with a higher risk of SCI (OR = 1.02, 95% CI: 1.01–1.04, p = 0.01), whereas no significant association was observed below the threshold ( p = 0.06). Conclusion In Chinese Han patients with AD, NLR was independently and linearly associated with a higher risk of SCI, while SII showed a threshold effect; PLR and SIRI exhibited no independent associations after multivariable adjustment. Among the four composite inflammatory indices, NLR and SII may hold potential value for assessing cognitive impairment severity in AD, whereas the negative findings for PLR and SIRI suggest that not all composite inflammatory indices are independently associated with SCI.