Other specialized tests that may be useful in assessing selected patients include analysis of synovial fluid, arthroscopy, and laboratory testing to detect possible infection.1
Synovial fluid analysis
Analysis of synovial fluid (the fluid inside of the joint capsule) can be useful in determining if a patient has gout or a form of infectious arthritis. The analysis typically involves a cell count as well as a search for uric acid crystals which are a sign of gout.1
To obtain synovial fluid, your doctor will insert a needle into one or more symptomatic joints and withdraw a small sample of synovial fluid. Withdrawal of fluid is also sometimes done to relieve joint pain due to pressure from build-up of fluid. This procedure is called joint aspiration, joint tapping, or is referred to by the technical term arthrocentesis.2
How is joint aspiration performed?
Joint aspiration is typically done using a medium gauge needle (22 gauge) or smaller for smaller joints. The procedure involves sterilization of the skin area where the needle will be inserted and use of a local anesthetic. A spray anesthetic, such as ethyl chloride is commonly used to numb the site of needle penetration. Your doctor may mark the joint site and have you flex the joint to make sure of the location for aspiration. While joint aspiration may be uncomfortable, it can usually be done quickly and only a small amount of fluid is withdrawn (as little as a drop or as much as a couple of tablespoons).2
What will my doctor be looking for in the synovial fluid?
Healthy synovial fluid is clear. Synovial fluid from a joint affected by RA may be cloudy, indicating a build-up of white blood cells (leukocytes) and other immune system chemicals. If there is damage to the joint, the fluid may have some blood in it.
In patients with RA, analysis of synovial fluid will often show cells counts that indicate inflammation, including elevated white blood cell counts ranging between 1,500 and 25,000 cells per cubic millimeter. If a patient has very active disease, white blood cell counts may be over 25,000 cells per cubic millimeter. Such a high cell count can also indicate the presence of infection. Other results of analysis that are characteristic of RA include low glucose (a type of sugar) and levels of immune system chemicals and proteins that indicate an immune response.3
Arthroscopy
Arthroscopy is performed by an orthopedic surgeon and involves inserting a fiberoptic camera (about the size of a small straw) through a small incision into the joint space to examine the interior of the joint. This procedure can be used to aid diagnosis and provide a means of repairing damage to joint tissues, including removing bone or cartilage, or repairing ligaments.
Arthroscopy is done by making a small incision in the area near the joint to be examined. The arthroscope camera (also called an endoscope) is inserted into the joint through the incision. A second incision can be made for insertion another tube that allows small surgical instruments to be used inside the joint. Arthroscopy provides an advantage over regular surgery in that the joint does not have to be opened up completely.
Laboratory testing
Laboratory tests of blood and other fluids, especially in patients who test negative for rheumatoid factor or anti-citrullinated protein antibodies, may be useful in detecting an infection, such as Lyme disease or hepatitis B virus (HBV) or hepatitis C virus (HCV).