New multi-inflammatory indices for evaluating disease activity in ankylosing spondylitis

Abstract Objective Ankylosing spondylitis (AS) is a chronic autoimmune rheumatic disease primarily affecting the axial joints, in which objective assessment of disease activity remains challenging. This study aimed to investigate the relationship between Multi-Inflammatory Index (MII) values — MII-1, MII-2, and MII-3 — and disease activity in patients with AS, and to evaluate their potential utility as complementary diagnostic tools. Methods This single-center, cross-sectional study included 114 AS patients fulfilling the Modified New York criteria and 60 age- and sex-matched healthy controls. MII-1 was calculated as neutrophil-to-lymphocyte ratio (NLR) × CRP, MII-2 as platelet-to-lymphocyte ratio (PLR) × CRP, and MII-3 as systemic immune-inflammation index (SII) × CRP. Disease activity was assessed using the BASDAI, with a score ≥ 4 defined as high disease activity. Results MII-1, MII-2, and MII-3 were significantly elevated in AS patients compared to controls (all p < 0.001). No statistically significant difference was observed between active and inactive groups regarding MII indices ( p > 0.05). PLR showed a weak but significant positive correlation with BASDAI ( r = 0.251, p = 0.007). ROC analysis demonstrated that CRP had the highest diagnostic performance (AUC = 0.746), followed by MII-2 and MII-3 (AUC = 0.714 for both), whereas NLR, PLR, and SII individually failed to achieve significant discriminatory ability. Conclusion MII indices did not demonstrate a statistically significant difference between active and inactive disease groups, indicating that their utility in monitoring disease activity requires further investigation. Nevertheless, MII-2 and MII-3 showed acceptable discriminatory performance in distinguishing AS patients from healthy controls. These findings suggest that composite inflammatory indices may offer modest complementary value in certain clinical contexts, but prospective studies with larger cohorts are needed before any clinical application can be recommended.