Classification criteria for psoriatic arthritis and ankylosing spondylitis/axial spondyloarthritis.

The concept of spondyloarthritides (or spondyloarthropathies, SpAs) that comprises a group of interrelated disorders has been recognised since the early 1970s. While the European Spondyloarthropathy Study Group (ESSG) criteria and the Amor criteria have been developed to embrace the entire group of SpAs, new criteria for psoriatic arthritis have been developed recently. The Classification of Psoriatic Arthritis (CASPAR) study, a large one of more than 1000 patients, led to a new set of validated classification criteria for psoriatic arthritis. Since their publication in 2006 the CASPAR criteria are widely used in clinical studies. In ankylosing spondylitis, the 1984 modified New York criteria have been used widely in clinical studies and daily practice but are not applicable in early disease when the characteristic radiographical signs of sacroiliitis are not visible but active sacroiliitis is readily detectable by magnetic resonance imaging (MRI). This led to the concept of axial SpA that includes patients with and without radiographical damage; candidate criteria for axial SpA were developed based on proposals for a structured diagnostic approach. These criteria were validated in the Assessment of Spondyloarthritis International Society (ASAS) study on new classification criteria for axial SpA, a large international prospective study. In this new criteria, sacroiliitis showing up on MRI has been given as much weight as sacroiliitis on radiographs, thereby also identifying patients with early axial SpA. Both the CASPAR and the ASAS criteria for axial SpA are likely to be of use as diagnostic criteria.