A procedure that involves the withdrawal of synovial fluid, the fluid that lubricates the surfaces of the bones and provides nutrients to the cartilage inside a joint. Fluid obtained from joint aspiration or arthrocentesis may be used as a diagnostic tool; an analysis of the fluid can help establish the cause of swelling in a joint, distinguish between different forms of arthritis, and detect the presence of blood in the joint, indicating trauma or fracture. The procedure can also help diagnose septic arthritis and joint diseases associated with the presence of crystals in the joint.