Assessment of peripheral arthritis and related disease activity measurement instruments in spondyloarthritis

Spondyloarthritis (SpA) is a chronic disease in which the spine and other areas of the body become inflamed. SpA can present in different physical and clinical forms such as axial SpA (axSpA) which mainly presents as back pain, or peripheral SpA (pSpA) which is characterized by inflammation and pain in joints located outside the spine. The two types can occur together or separately.

A clinician’s instrument called the ASAS-OMERACT core outcome set (The Assessment of Spondylarthritis international Society-Outcomes Measures in Rheumatology core outcome set (COS)) has recently been updated to allow it to be used to assess axSpA, however for pSpA the 44-swollen joint count (SJC44), which looks at 44 joints in the body to assess swelling, was chosen as the preferred instrument recommended for use in clinical trials, but no combination of scoring systems (composite scores - tools that combine several different measures or tests into one overall score) were assessed in that project.

Although scarce, there is some evidence of the good performance of composite scores when assessing disease activity in the pSpA population. For example, in a study addressing the sensitivity to change and discriminatory aspects of different measurement instruments (capacity to discriminate between different groups and scenarios), the combination of Axial Spondyloarthritis Disease Activity (ASDAS), Bath Ankylosing Spondylitis Disease Index (BASDAI), patient global assessment (PGA), and physician global assessment (PhGA) had the highest performance, whereas the SJC, tender joint count (TJC) and C-reactive protein (CRP) performance was not good enough. In another attempt at assessing the validity of measurement instruments in pSpA, good performance was found for the Disease Activity Index for Psoriatic Arthritis (DAPSA), Psoriatic Arthritis (PsA) Disease Activity Score (PASDAS) and Ankylosing spondylitis disease activity score (ASDAS), although the agreement among the three instruments when classifying disease activity states was not sufficient. This finding emphasized the need of further analysis of the existing thresholds (the established cut-off values used to categorize a score into different disease activity states) of those instruments in pSpA.

Recently, a study was conducted, called ASAS-perSpA, which included patients with axSpA, pSpA and PsA, and the validity of several measurement instruments was assessed, including the SJC, but also different composite scores validated for SpA and even in other inflammatory diseases such as the Disease Activity Score (DAS) 28 and 44. Results from this analysis showed that composite scores including joint counts have superior construct (is better at measuring what it is intended to measure) by discriminating better between active/inactive patients. Simultaneously, the psychometric properties of 9 measurement instruments were assessed in the CRESPA trial. In this study, conducted in patients with pSpA, both the ASDAS and DAPSA showed good validity and clinical trial discrimination, with DAPSA exhibiting numerically better results.

The aim of this study is to compare the construct validity (the degree to which an outcome measure measures the construct it purports to measure) and discrimination of different measurement instruments to assess disease activity due to peripheral arthritis in patients with spondyloarthritis (either axSpA or pSpA), with the aim of selecting the most adequate and best performing measurement instrument to be endorsed by Assessment of SpondyloArthritis International Society (ASAS).