Joint fluid analysis can help determine the cause of joint pain or inflammation. This procedure may also relieve pain and swelling caused by joint inflammation.

Normally there is only a small amount of joint fluid (also called synovial fluid) in a joint. Joint fluid acts as a lubricant for the joint and cushions joint structures. If the joint is infected or injured, the amount of fluid may increase and the joint may become swollen, stiff, and painful. Removing a sample of joint fluid and analyzing it can help determine the cause of joint pain and swelling.

For joint fluid analysis, a sample of joint fluid is removed from the joint with a long, thin needle. This procedure is performed most often on the knee, but it can also be done on other major joints in the body, including the 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.

Joint Fluid Analysis
Results

Joint fluid analysis includes examining the fluid under a microscope for the presence of bacteria, crystals, and blood cells. Rarely, glucose, protein, and enzymes (such as lactic dehydrogenase, or LDH) may also be measured. Results of this test may indicate:

• Diseases caused by the build up of crystals (gout and pseudogout).
• A bacterial infection.
• Noninflammatory diseases, most commonly osteoarthritis.
• Inflammatory diseases, most commonly rheumatoid arthritis, psoriatic arthritis; less commonly systemic lupus erythematosus.

Abnormal values may mean

• Color and clarity. Deep, dark red color may be a sign of bleeding in the joint. Milky white color may indicate infection or inflammation.
• Blood cell count. Large numbers of red blood cells may indicate bleeding in the joint caused by injury, inflammation, or abnormal clotting of the blood. Large numbers of white blood cells may indicate gout, pseudogout, inflammatory forms of arthritis such as rheumatoid arthritis, or infection.
• Presence of crystals. Uric acid crystals in the joint indicate gout. Calcium pyrophosphate crystals indicate pseudogout.
• Gram stain and culture. Bacteria seen under a microscope on the Gram stain (a special dye) of the joint fluid strongly suggests the presence of infection. Bacteria that grows out of a culture plate in 1 to 2 days confirms the presence of an infection.
• Glucose. Low levels of glucose in the joint fluid may indicate infection or severe inflammation. However, when the level of joint fluid glucose is abnormal, the level of joint fluid glucose needs to be compared to the blood glucose level.
• Lactic dehydrogenase (LDH). High levels of LDH in the joint fluid and normal blood levels of LDH may indicate rheumatoid arthritis, gout, or infection.
• Protein. High protein levels in the joint fluid may indicate inflammation or infection.

After your doctor has determined the cause of your joint pain, he or she may recommend further treatment with medication, physical therapy, or surgery.