PsA is similar to RA however the lab work used to show you have RA. spicifically rhumatoid factor is negative in people with PsA.
I was in the lad one with a woman who told me she had PsA and RA, I didn't think that was possible!
Autoimmune diseases are very complex and hard for Dr.'s to know the initial triggor because the immune system has so many triggers and is quite amazing.
Specifically in PsA it involves the following factors
Psoriatic arthritis (PsA) is a chronic inflammatory arthropathy of unknown etiology that is associated with psoriasis. Joint destruction is often progressive: almost half of the patients attending an early arthritis clinic showed radiological damage 2 years after diagnosis. Proinflammatory cytokines are major mediators of systemic and local inflammation, and high levels of interleukin 1 (IL-1), IL-6, and tumor necrosis factor have been found in psoriatic skin lesions and the synovial tissue of patients with rheumatoid arthritis and PsA. IL-6 is a pleiotropic cytokine that mainly signals by membrane (neutrophil and lymphocyte) or soluble (endothelial cell) IL-6 receptors. IL-6 was originally identified as a factor in B cell differentiation, but is now known to influence T cell development: in the presence of IL-6 and transforming growth factor-ß (TGF-ß), naive T cells develop into Th17 cells, which are important mediators in PsA. IL-6 may also directly contribute to the epidermal hyperplasia seen in psoriatic epithelium and affect the function of dermal inflammatory cells. However, there are no data concerning the use of tocilizumab in patients with PsA, although a pilot study is currently being carried out because the role of IL-6 in the pathogenesis of PsA supports the idea that targeted treatments against IL-6 might be effective.
In Rheumatoid Arthritis a major indicator; RA level is often evaluated in patients suspected of having any form of arthritis even though positive results can be due to other causes, and negative results do not rule out disease. But, in combination with signs and symptoms, it can play a role in both diagnosis and disease prognosis. It is part of the usual disease criteria of rheumatoid arthritis.
The presence of rheumatoid factor in serum can also indicate the occurrence of suspected autoimmune activity unrelated to rheumatoid arthritis, such as that associated with tissue or organ rejection. In such instances, RF may serve as one of several serological markers for autoimmunity.[5]
Rheumatoid factor (RF) is the autoantibody (antibody directed against an organism's own tissues) that is most relevant in rheumatoid arthritis.[1] It is defined as an antibody against the Fc portion of IgG. RF and IgG join to form immune complexes that contribute to the disease process.[citation needed]
Rheumatoid factor can also be a cryoglobulin (antibody that precipitates on cooling of a blood sample); it can be either type 2 (monoclonal IgM to polyclonal IgG) or type 3 (polyclonal IgM to polyclonal IgG) cryoglobulin.
Rheumatoid factor can be of any isotype of immunoglobulins, i.e. IgA, IgG, IgM,[2] IgE,[3] IgD.[4]
Unfortunately PsA was previously though to be less aggressive than RA, however now that studies have shown that it can be quite devastating.
But in the case of people asking what you have its easier to say its like it like RA.
Hope that helps.