Objectives To develop a preliminary composite psoriatic disease activity index (CPDAI) for psoriasis and psoriatic arthritis.

Methods Five domains were assessed and specific instruments were employed for each domain to determine the extent of domain involvement and the effect of that involvement on quality of life/function. Disease activity for each domain was then graded from 0 to 3 giving a CPDAI range of 0–15. Patient and physician global disease activity measures were also recorded and an independent physician was asked to indicate if treatment change was required. Bivariate correlation analysis was performed. Factor, tree analysis and standardised response means were also calculated.

Results Significant correlation was seen between CPDAI and both patient (r=0.834) and physician (r=0.825) global disease activity assessments (p=0.01). Tree analysis revealed that 96.3% of patients had their treatment changed when CPDAI values were greater than 6; no patient had their treatment changed when CPDAI values were less than 5.

Conclusion CPDAI correlates well with patient and physician global disease activity assessments and is an effective tool that clearly distinguishes those who require a treatment change from those who do not.