The Systemic Immune‐Inflammation Index and Its Association With Biologic Therapy Switching and Response in Psoriasis

ABSTRACT Background Biologic therapies have revolutionised psoriasis treatment by targeting immune pathways. Despite these advances, some patients experience suboptimal outcomes, necessitating frequent therapeutic switches. The systemic immune‐inflammation index (SII) is a novel biomarker for systemic inflammation. Its role in biologic switching in psoriasis remains unexplored. Objectives To evaluate the association between baseline SII levels and biologic therapy switching in patients with moderate‐to‐severe psoriasis. Methods This single‐centre retrospective cohort study included 98 patients with moderate‐to‐severe psoriasis initiating biologic therapy in 2017. Patients were followed for 5 years to document biologic switches. Baseline demographics, disease severity, and SII levels were analysed. Statistical comparisons and multinomial logistic regression assessed associations between SII and biologic switching. Results Of the 98 patients, 48 (49%) switched biologics during follow‐up: 23 (23.5%) made a single switch, and 25 (25.5%) were multiple switchers (≥ 2 biologics). Baseline SII was significantly higher in switchers compared to non‐switchers ( p < 0.001). Mean SII values were 560 (95% CI: 456–663) for non‐switchers, 1068 (95% CI: 915–1220) for single switchers, and 1275 (95% CI: 1129–1421) for multiple switchers. Higher PASI, DLQI, psoriatic arthritis (PsA) prevalence, smoking, and body weight were also linked to switching ( p < 0.05). Conclusions Elevated baseline SII is strongly associated with biologic switching, particularly among multiple switchers, highlighting its potential as a stratification tool. Integrating SII into psoriasis treatment algorithms may enable clinicians to identify high‐risk patients early, optimise biologic selection, and improve long‐term outcomes. This study underscores the importance of incorporating inflammatory markers into personalised psoriasis management strategies.