Psoriatic arthritis is a "type of inflammatory arthritis associated with psoriasis, often involving the axial joints and the peripheral terminal interphalangeal joints. It is characterized by the presence of HLA-B27-associated spondylarthropathy, and the absence of rheumatoid factor" (RF).
The original diagnostic criteria, not involving genetic analysis, are of Moll & Wright: Rheumatologists find these still somewhat vague, and also can be confounded by comorbidities. For example, a patient might have rheumatoid arthritis and have a positive rheumatoid factor test, but coincidentally have non-arthritic psoriasis.  There also may be hidden psoriatic lesions, and Helliwell & Taylor suggest that confirmed psoriasis in a first-degree relative may meet the criteria.
- Anonymous (2015), Psoriatic arthritis (English). Medical Subject Headings. U.S. National Library of Medicine.
- Moll JMH, Wright V. (1973), "Psoriatic arthritis", Semin Arthritis Rheum (no. 3): 55–78
- P S Helliwell, W J Taylor (2005), "Classification and diagnostic criteria for psoriatic arthritis", Ann Rheum Dis (no. 64(Suppl II)): ii3–ii8., DOI:10.1136/ard.2004.032318