Synovial fluid analysis Arthrocentesis Joint aspiration
Removal of joint fluid has a dual purpose—diagnostic and therapeutic. While finding the cause of joint swelling is the goal, the removal of fluid may also help to relieve pain and pressure on the joint.
The Technique
Aseptic technique must be followed when obtaining synovial fluid for analysis. Precautions are taken so that bacteria are not introduced into the joint. A sterile needle is used to draw joint fluid from the affected joint, but first, the skin is sterilized using a topical agent, e.g., Betadine (povidone-iodine). A local anesthetic is also used.
After the fluid is withdrawn from the joint, the healthcare provider may inject medicine (usually a corticosteroid) into the joint using the same injection site. A bandage is applied after the needle has been removed.
Appearance
Once the joint fluid arrives at the laboratory for analysis, it is observed by the human eye for color and clarity. Normal joint fluid is viscous and appears clear to light yellow. Cloudy joint fluid is abnormal and suggestive of inflammation or infection. Bloody joint fluid is also abnormal and may be caused by trauma to the joint.
Microscopic Examination
Joint fluid is examined under a microscope and blood cell counts are done.
Normal joint fluid has none or few blood cells. Large numbers of red blood cells indicate bleeding in the joint. Large numbers of white blood cells can occur with infection, inflammatory arthritis, gout, or pseudogout.
Typically, a high white blood cell count suggests inflammation and a very high white blood cell count suggests an infection.
A synovial fluid culture is an important test that takes a sample of the joint fluid and examines it for infection-causing germs, such as bacteria. A culture can identify the source of a bacterial infection.
Crystals seen under a microscope are an abnormal finding. Uric acid crystals are indicative of gout while CPPD crystals occur with pseudogout.
Chemical Analysis
Although rarely used, joint fluid can also be tested for glucose, protein, and lactic dehydrogenase (LDH). Abnormal joint fluid results may indicate inflammation or infection.
Other Findings From Synovial Fluid Analysis
Joint fluid is observed in a plain tube after one hour for the formation of a fibrin clot. The quality of the clot is graded, but any clot suggests that there is a problem with the synovial membrane.
Blood tests or other laboratory tests can also be ordered with the intent of supporting the evidence derived from the synovial fluid analysis.
