Test Overview
Joint fluid analysis is a test to examine joint fluid under a microscope for the presence of bacteria, crystals, and blood cells. The test can help determine the cause of joint pain or inflammation.

Normally only a small amount of joint fluid (also called synovial fluid) is found in a joint. Joint fluid acts as a lubricant for the joint and cushions joint structures. See an illustration of the joint. If your joint is infected, injured, or inflamed, the amount of fluid may increase and your joint may become swollen, stiff, and painful.

A sample of joint fluid is usually removed from your knee, but it can also be done on other major joints in your body, including your shoulder, hip, elbow, wrist, or ankle. The joint fluid is then analyzed in a lab to look for signs of infection, bleeding, or other causes of inflammation.