OBJECTIVES: The Disease Activity index for Psoriatic Arthritis (DAPSA) was developed to assess disease activity in patients with psoriatic arthritis (PsA). A modified version, DAPSA28, uses 28 joints instead of 66/68. This study evaluated key psychometric properties of DAPSA and DAPSA28. METHODS: Data from 1865 patients with PsA in the European Spondyloarthritis (EuroSpA) Research Collaboration Network, having DAPSA and DAPSA28 scores at baseline and follow-up, were analysed. Tests included assessment of internal construct validity by scree plots, confirmatory factor analysis (CFA) and structural equation modelling (SEM), supplemented by tests of differential item functioning (DIF) and evaluation of internal consistency reliability by Cronbach's α (CA). A subset of 625 patients was used for most analyses, except descriptive statistics, correlation matrix and CA. RESULTS: One-dimensional CFA models for DAPSA and DAPSA28 showed acceptable model fit at baseline (root mean square error of approximation, RMSEA: 0.020, 0.034). However, model fit at 6 months follow-up was poor (RMSEA: 0.057, 0.063). SEM combining baseline and follow-up data could not identify an acceptable model fit. DIF was found for sex and country. CA indicated acceptable internal consistency (DAPSA: 0.65; DAPSA28: 0.63). Heterogeneity across countries was observed. CONCLUSIONS: Overall, the model fit was acceptable across model fit statistics, supporting internal construct validity, but some evidence of misfit at country level was disclosed. Our findings support acceptable internal consistency reliability, but DIF was found for sex and country. Based on mixed results of model fit and DIF, further investigation of these and other PsA disease activity measures is warranted.

Center for Rheumatic Diseases University of Medicine and Pharmacy Bucharest Romania

Center for treatment of Rheumatic and Musculoskeletal Diseases Diakonhjemmet Hospital Oslo Norway

Centre for Rheumatology Research Landspitali University Hospital Reykjavík Iceland

Copenhagen Center for Arthritis Research Center for Rheumatology and Spine Diseases Rigshospitalet Glostrup Denmark

Copenhagen Research Center for Biological and Precision Psychiatry Mental Health Centre Copenhagen Copenhagen University Hospital Copenhagen Denmark

DANBIO Rigshospitalet Glostrup Denmark

Department of Clinical Internal Anesthesiological and Cardiovascular Sciences Sapienza University of Rome Rome Italy

Department of Clinical Medicine Aarhus University Aarhus Denmark

Department of Clinical Medicine University of Copenhagen Copenhagen Denmark

Department of Internal Medicine University of Turku Turku Finland

Department of Public Health University of Copenhagen Copenhagen Denmark

Department of Rheumatology 1st Faculty of Medicine Charles University Prague Czech Republic

Department of Rheumatology and DANBIO Aarhus University Hospital Aarhus Denmark

Department of Rheumatology Division of Medicine Turku University Central Hospital Turku Finland

Department of Rheumatology East Tallinn Central Hospital Tallinn Estonia

Department of Rheumatology Geneva University Hospitals Geneva Switzerland

Department of Rheumatology University Hospital Zurich University of Zurich Zürich Switzerland

Departments of Medicine and Rheumatology Helsinki University Central Hospital Helsinki Finland

Faculty of Health and Medical Sciences University of Copenhagen Copenhagen Denmark

Faculty of Medicine University of Iceland Reykjavík Iceland

Institute of Rheumatology Prague Czech Republic

Instituto Português de Reumatologia Lisbon Portugal

Landspitali University Hospital Reykjavík Iceland

Research Unit Sørlandet Sykehus HF Kristiansand Norway

Rheumatology Unit DiMePReJ University of Bari Bari Italy

University of Iceland Faculty of Medicine Reykjavík Iceland

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