Diagnosis, classification, and assessment in psoriatic arthritis.

There have been considerable advances in the classification and assessment of psoriatic arthritis (PsA). In this report, we give an overview of historic and current classification criteria and discuss its role and limitations in research and clinical practice. We discuss the most commonly used assessment instruments for arthritis, psoriasis, onychodystrophy, enthesitis, dactylitis and axial PsA with a focus on clinical practice. We pay particular attention to the current evidence for the use of composite outcome measures, and their use in randomised controlled trials and routine care.