When a joint swells, there is accumulation of fluid inside, where there is usually just a thin layer of lubricating fluid. The fluid may be blood, if the swelling is the result of an injury, or it may be excess synovial fluid as in a flare of arthritis. It may also be purulent in the face of an infection, or contain inflammatory cells that may help diagnose a more systemic problem. Joint aspiration can be performed after instillation of a local anesthetic agent into the skin to relieve pressure on the joint thereby relieving pain, to help diagnose an infection, to send fluid for microbacterial or pathologic evaluation (to diagnose a condition like gout). Ultrasound or fluoroscopy can be used to guide the needle placement if necessary.