Evaluation of instruments assessing peripheral arthritis in spondyloarthritis: an analysis of the ASAS-PerSpA study

To assess construct validity, including known-group discrimination, of the currently available disease activity instruments assessing peripheral arthritis in spondyloarthritis (SpA).

In this analysis from the Assessment of SpondyloArthritis International Society (ASAS)-PerSpA study, patients with a diagnosis of axial SpA, peripheral SpA, or psoriatic arthritis (PsA) were included. The disease activity instruments evaluated were the Patient Global Assessment (PGA), Bath Ankylosing Spondylitis Disease Activity Index, Axial Spondyloarthritis Disease Activity Score, Disease Activity Index for PsA (DAPSA), Swollen Joint Count (SJC), Tender Joint Count, Disease Activity Score (DAS) 28, DAS44, and C-reactive protein (CRP). Construct validity was assessed through correlations with external constructs (Bath Ankylosing Spondylitis Functional Index, ASAS Health Index, and Euro Quality of Life 5 Dimensions) and known-group discrimination (active/inactive disease based on a combination of PGA [≥5/<5]), and SJC (≥1/0 and ≥2/<2) was analysed using standardised mean differences (SMDs).

In total, 4121 patients were included (mean age 45 [SD, 14] years, 61% males). When assessing the construct validity through correlations with external constructs, all instruments performed excellently (100% hypotheses confirmed). When assessing known-group discrimination, all disease activity measures, except CRP, presented SMDs ≥ 0.8 (good discrimination), with higher SMDs observed for DAS28 followed by DAPSA. Results were similar across disease phenotypes. Considering all combinations of PGA and SJC to discriminate between active/inactive disease, a better performance was observed for the composite scores, including joint counts.

In our construct validity analysis, all disease activity instruments assessing peripheral arthritis had a good performance as reflected in the correlations with external constructs and the known-group discrimination. The highest discriminatory capacity to distinguish between 'active/inactive disease' was observed for composite scores, including joint counts, like DAS28 and DAPSA.