Often a sample of blood or joint fluid drawn from the knee 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. A blood test that detects an immune response to the infectious agent that causes Lyme disease could be helpful in confirming a diagnosis of that disease.
Tests of fluid drawn from the knee joint with a needle may reveal crystals of uric acid, confirming a diagnosis of gout, or bacteria, suggesting that joint inflammation is caused by an infection.