Osteoarthritis can almost always be diagnosed without the need for laboratory tests, which are routinely normal in osteoarthritis patients. The main reason for laboratory tests is to rule out other possible diseases such as rheumatoid arthritis. For this reason, some doctors routinely order two blood tests for all patients with painful joints: the rheumatoid factor test and erythrocyte sedimentation. These tests can help to indicate whether rheumatoid arthritis is present.
Taking a sample of synovial fluid, removed from the joint with a needle, also helps rule out other possible health problems. Abnormally high levels of white blood cells indicate inflammation and the presence of several possible conditions — including gout, inflammatory types of arthritis such as rheumatoid arthritis or psoriatic arthritis, or arthritis due to an infection (septic arthritis). As a bonus, draining fluid from a joint can help relieve pressure and pain in a joint.