Comparative analysis of instruments measuring peripheral arthritis activity in Spondyloarthritis: a measurement properties study in 13 RCTs.

OBJECTIVES: Using randomised controlled trial (RCT) data, this study evaluated the measurement properties of selected instruments to assess peripheral arthritis in axial and peripheral spondyloarthritis (axSpA/pSpA) and to inform selection of the best-performing instrument. METHODS: Eligible studies were RCTs in axSpA or pSpA with individual patient data available to calculate Patient Global Assessment (PGA), swollen/tender joint counts (44 or 66/68 joints), C reactive protein, Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Axial Spondyloarthritis Disease Activity Score, Disease Activity Index for Psoriatic Arthritis/Disease Activity in Psoriatic Arthritis with 44 joint count (DAPSA44) and 44-joint Disease Activity Score at baseline and timing of primary endpoint. In axSpA, analyses were restricted to patients with peripheral arthritis at baseline. Measurement properties assessed included construct validity, test-retest reliability, longitudinal construct validity, trial discrimination and thresholds of meaning. Risk of bias (RoB) was evaluated and evidence was synthesised using Outcome Measures in Rheumatology-based GREEN/AMBER/RED ratings (low/uncertain/high RoB and good/adequate/poor performance, respectively). RESULTS: 13 RCTs were included (10 axSpA and 3 pSpA). In axSpA trials, 25-51% of patients had peripheral arthritis at baseline (median swollen joint count of 44 joints (SJC44) across studies: 2-3); in pSpA trials, median SJC44 ranged 2-5. In axSpA, construct validity and thresholds of meaning were rated AMBER/GREEN for all instruments, while test-retest reliability was RED only for PGA. Responsiveness was consistently GREEN. Clinical trial discrimination was RED for single-item instruments but consistently GREEN for composite scores. In pSpA, most instruments showed AMBER/GREEN performance, although SJC44 demonstrated RED discrimination. CONCLUSIONS: This study provides the first comprehensive, trial-based evaluation of measurement properties for instruments assessing peripheral arthritis activity in axSpA and pSpA. Composite scores, such as BASDAI, ASDAS and DAPSA44, generally demonstrated more robust performance than single-item instruments, expanding the evidence base for the final instrument-selection phase.