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Psoriatic arthritis (PsA) is an inflammatory musculoskeletal disease that affects people with psoriasis or their near relatives. It affects musculoskeletal structures such as the peripheral and axial joints, entheses, and tendon sheaths. The eye and mucous membranes are also often involved. Thus, the disease has varied manifestations that make diagnosis and assessment sometimes difficult.
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The original case definition for PsA was provided by Moll and Wright in 1973.1 They defined PsA as an inflammatory arthritis associated with cutaneous psoriasis, seronegative for rheumatoid factor. Rheumatoid factor is a marker for rheumatoid arthritis (RA); thus, the definition was meant to help distinguish PsA from RA, which at that time was a more recognized form of inflammatory arthritis. The CASPAR study group recently developed a new set of criteria for classification of PsA using data collected prospectively in patients with long-standing disease (Box 19-1).2 The CASPAR criteria had specificity of 98.7% and sensitivity of 91.4% in the original study, with excellent sensitivity in both early and late disease. The criteria allow classifying patients even when they do not have current, past, or family history of psoriasis and are now used in epidemiologic and genetic studies in PsA.
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