Proposal for a disease severity score for axial spondyloarthritis: results from an Italian longitudinal study

OBJECTIVES: Effective assessment of disease severity in axial spondyloarthritis (axSpA) is essential for clinical decision-making, particularly in patients naive to biologic therapy. We developed the Spondyloarthritis Severity Index (SpA-SI), a composite tool designed to capture key clinical and inflammatory domains of axSpA, and evaluated its performance in a prospective cohort. METHODS: One hundred bDMARD-naive patients with axSpA were enrolled. The SpA-SI integrates six parameters: FABER test, morning stiffness, VAS for back pain, extraarticular manifestations, peripheral synovial and entheseal involvement, and CRP levels. The total score ranges from 0 to ≥7 points. RESULTS: At baseline, the median SpA-SI score was 7.2 (IQR 6.2-7.6; range 3.4-12.2), which significantly improved to 4.9 at 6 months (IQR 2.8-7.0; range 1.2-9.5; p<0.001). SpA-SI showed a strong correlation with ASDAS both as a continuous measure and in categorised disease activity groups (p<0.001). Moderate-to-strong correlations with BASDAI were observed at baseline (r=0.47, p<0.001) and at six months (r=0.79, p<0.001), with similar significance when analysed by group. CONCLUSIONS: The SpA-SI represents an exploratory, multidimensional index that captures key clinical and inflammatory domains. By incorporating extra-articular manifestations and physical examination findings, it reflects the complexity of axSpA in routine care. Despite its limitations, including single-centre design and absence of formal validation, the SpA-SI demonstrated strong correlations with established indices and sensitivity to change. These findings support further validation in larger, multicentre cohorts to assess its potential role in complementing existing disease activity measures.