The systemic immune-inflammation index (SII) is a recently developed indicator for systemic inflammatory response. We aimed to explore the association between SII and disease activity in patients with ankylosing spondylitis (AS).
This retrospective study included 136 patients with AS and 63 healthy controls. Patients were divided into two groups according to Bath Ankylosing Spondylitis Disease Activity Index (BASDAI); active group (n = 60) and remission group (n = 76). Clinical, laboratory, and demographic characteristics were recorded. Spearman’s correlation analysis was used to determine correlations of SII with C-reactive protein (CRP) level, erythrocyte sedimentation rate (ESR), and BASDAI in AS patients. Binary logistic regression analysis was used to assess risk factors for AS disease activity. Receiver operating characteristic curve analysis was used to evaluate the diagnostic value of SII and the above variables for the active group compared with the remission group.
Systemic immune-inflammation index levels were higher in AS patients than in healthy controls (p < 0.001). SII levels were higher in the active group than in the remission group (p < 0.001). For patients with AS, SII correlated positively with CRP (r  = 0.483, p < 0.001), ESR (r  = 0.374, p < 0.001), and BASDAI (r  = 0.667, p < 0.001). SII (OR = 1.009, 95% CI = 1.006-1.012, p < 0.001) was an independent risk factor affecting AS disease activity. The specificity and sensitivity of SII using a cutoff value of 513.2 were 83.33% and 86.84%, respectively, for the active group.
Systemic immune-inflammation index was increased in AS. SII may be a novel indicator for monitoring AS disease activity.

© 2021 The Authors. Journal of Clinical Laboratory Analysis published by Wiley Periodicals LLC.