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How does a doctor diagnose psoriatic arthritis?
Psoriatic arthritis is a diagnosis made mainly on clinical grounds, based on the finding of psoriasis and the typical inflammatory arthritis of the spine and/or other joints. There is no laboratory test to diagnose psoriatic arthritis. Blood tests such as sedimentation rate may show an abnormal elevated result and merely reflect presence of inflammation in the joints and other organs of the body. Other blood tests, such as rheumatoid factor, are obtained to exclude rheumatoid arthritis. When one or two large joints (such a knees) are inflamed, arthrocentesis can be performed. Arthrocentesis is an office procedure whereby a sterile needle is used to withdraw (aspirate) fluid from the inflamed joints. The fluid is then analyzed for inflammation, infection, gout crystals, and other inflammatory conditions. X-rays may show changes of cartilage or bone injury indicative of arthritis of the spine, sacroiliac joints, and/or joints of the hands. Typical X-ray findings include bony erosions resulting from arthritis, but these may not be present in early disease. MRI scanning is sometimes used to identify early erosion of joints. The blood test for the genetic marker HLA-B27, mentioned above, is often performed. This marker can be found in over 50% of patients with psoriatic arthritis who have spine inflammation.