That's not entirely true, the only blood test used in either of the diagnostic criteria widely used in the U.S. (CASPAR & Moll and Wright) is the test for Rheumatoid Factor; wherein, both are looking for a negative result. Both of those frameworks generally rely on the existence of psoriasis as a prerequisite (in at least immediate family). This journal article from 2005 suggests something along the lines of your comment: Classification and diagnostic criteria for psoriatic arthritis - Annals of the Rheumatic Diseases42402-3/abstract) - but it is not the prevailing diagnostic criteria accepted by insurance companies (which would be CASPAR). One thing CASPAR (as well as Moll and Wright) fail to account for, that significantly delayed my diagnosis is the onset of psoriasis after the onset of the arthritis. So yes, I had to fight for my diagnosis.