Disease activity indices in psoriatic arthritis: current and evolving concepts

Psoriatic arthritis (PsA) is a highly heterogenous disease, with many different clinical manifestations inside or outside of the musculoskeletal system and the skin. It is often accompanied by comorbidities like cardiovascular diseases and mental health disorders. Acute phase reactants are not always elevated and specific markers for diagnosis and/or monitor the disease are lacking thus far. These characteristics possibly reflect the difficulty in agreement about a disease activity index for PsA. Many indices have been proposed over the last years, each of them considering different combinations of disease characteristics. We performed a literature search for relevant articles using PubMed and Embase. No data limits were applied. The keywords "Psoriatic arthritis" OR "PsA" AND "disease activity" AND "index" OR "indices" were used. Reference lists of relevant articles were also reviewed. Articles were also identified through searches of the authors' own files. In this review, we comparatively present the available indices (simple or composite) used for measuring activity in PsA, highlighting their weaknesses, strengths, and disparities. We comment also on the caveats and pitfalls that are encountered in assessment of disease activity, in relation to clinical practice and research. A widely accepted index for measuring disease activity in PsA is lacking. Other parameters, mostly related to patient-reported outcomes and to novel biomarkers might be included in the future, in such indices.