Disease activity assessment in patients with psoriatic arthritis.

Psoriatic arthritis (PsA) is an inflammatory disease in patients with a personal or family history of psoriasis. While synovitis is the major hallmark, enthesitis, dactylitis, axial inflammation, and skin and nail involvement are variable in prevalence and severity. The assessment of disease activity is important to determine the treatment target ("Treat-to-Target") as well as early response to therapy, because a prolonged ineffective medication may not only be associated with an adverse disease outcome but constitutes a waste of individual and/or societal expenses. Various measures of disease activity have been established for all manifestations individually ('uni-dimensional approach') or by combining such measures into a single umbrella score ('multi-dimensional approach'). Because pathogeneses and therapeutic responsiveness differ among individual domains, a uni-dimensional approach is preferred. The major uni-dimensional index is the Disease Activity index for PSoriatic Arthritis, which provides a continuous scale and allows calculating disease activity at any point in time, determining response to therapy and defining disease activity states, such as remission, as a treatment target. The assessment of the various domains by individual and combined indices is discussed.