The diagnosis of psoriatic arthritis is mainly based upon a spectrum of characteristic findings noted in the history, physical examination, and radiographic findings. Psoriasis is typically present prior to, or at the onset of joint disease. Only a minority of individuals will have characteristic joint disease before psoriasis.
Psoriatic arthritis may be misdiagnosed as rheumatoid arthritis, osteoarthritis, or gout. Rheumatoid arthritis generally involves joints that are symmetrical on both sides of the body; however, some forms of psoriatic arthritis look very similar. One way to potentially differentiate between the two is having characteristic psoriatic skin or nail changes. Usually, patients with early signs of psoriatic arthritis will have had psoriasis for several years when they begin to feel discomfort in the joints.
There are no tests that are specific for the diagnosis of psoriatic arthritis. Laboratory findings of HLA-B27 positivity, and blood test signs of inflammation (elevated C reactive protein or erythrocyte sedimentation rate) are compatible but not specific for psoriatic arthritis.
Plain x-ray films of the lower spine will show definite signs of arthritis in the sacroiliac joints when affected.
Silman AJ, Hochberg MC. Epidemiology of the rheumatic diseases. 2nd edition: Oxford University Press; 2001.
Helmick CG, Felson DT, Lawrence RC et al. Estimates of the prevalence of arthritis and other rheumatic conditions in the United States. Part I. Arthritis & Rheumatism. 2008;58:15-25.
National Institutes of Health, National Institute of Arthritis and Musculoskeletal and Skin Disease. Handout on Health: Arthritis.