Often a sample of blood or joint fluid can help your doctor confirm a diagnosis. For example, a blood test showing high blood levels of rheumatoid factor – an antibody that acts against the blood component gamma globulin – or antibody called anti-cyclic citrullinated peptide antibody (anti-CCP) may suggest rheumatoid arthritis. High levels of antinuclear antibodies (ANAs), abnormal antibodies directed against the cells' nuclei, could suggest lupus or another inflammatory disease.  

For people with arthritis of the spine, a finding of a specific genetic marker called HLA-B27 in the blood can help the doctor identify a possible diagnosis of a spondylarthropathy, such as ankylosing spondylitis or reactive arthritis. Although the genetic marker is more common in people with these diseases, perfectly healthy people can have it. For that reason, a positive HLA-B27 test does not mean you have one of these diseases.

Tests of fluid drawn from the joint with a needle may reveal crystals of uric acid, confirming a diagnosis of gout, or a bacterium, suggesting that joint inflammation is caused by an infection.  

Learn more about lab tests used in diagnosing arthritis.