Background The Disease Activity Index for Psoriatic Arthritis (DAPSA) is a valid and discriminative tool. Definitions of disease activity states and therapeutic response are still missing. We derived such criteria for the DAPSA.

Methods We retrieved 30 patient profiles from an observational database including joint counts, patient pain and global activity ratings and C-reactive protein (CRP) and carried out a survey among experts to classify patients into remission (REM), low (LDA), moderate (MDA) or high (HDA) disease activity. Based on the distributions of DAPSA in each of these expert-assigned states we defined the cutpoints between groups. We performed similar analyses evaluating a clinical score (cDAPSA), omitting CRP. To define minor, moderate and major treatment response, we used Cohen's Kappa statistics and analysed agreement of DAPSA percentage change with ACR20/50/70-response in three randomised controlled trials.

Results Our survey yielded a response rate of 75% (n=33). Mean DAPSA differed significantly between patients classified as REM, LDA, MDA or HDA (p<0.001). Based on the distributions of DAPSA in these groups, we propose cut-off values of ≤4 for REM, >4 and ≤14 for LDA, >14 and ≤28 for MDA and >28 for HDA. We observed best agreement with ACR20/50/70-response at DAPSA changes of 50/75/85%, reflecting minor, moderate and major improvement.

Conclusions The DAPSA constitutes a disease-specific, validated and feasible tool for PsA assessment. In this study, we provide criteria for disease activity states and treatment response. They are based on an international expert survey, and show good performance in clinical trials and observational data.