Often a sample of blood or joint fluid can help you 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 an 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. A high erythrocyte sediment rate (ESR, or sed rate), along with muscle pain in the hips and shoulders and a skin rash over the knees and elbows could help confirm a diagnosis of dermatomyositis.

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

Learn more about lab tests used in diagnosing arthritis.