Psoriatic arthritis is a clinical diagnosis. The diagnosis is based on history of skin psoriasis and physical examination. The physical examination is to find clues of inflammation in a joint or the surrounding areas. Your rheumatologist could ask for blood testing and x-rays to rule out other causes of arthritis. These tests include markers of inflammation like sedimentation rate, also called ESR and CRP. Testing for blood markers of rheumatoid arthritis could help rule out this as a possibility.