What is being tested?
Synovial fluid is a viscous liquid that acts as a lubricant for the major joints of the body. It is found in small quantities in the spaces between the joints, where the fluid is produced and contained by synovial membranes. Synovial fluid cushions the bone ends and reduces friction during joint movement in the knees, shoulders, hips, hands and feet.
Synovial fluid analysis consists of a group of tests that detect changes in synovial fluid that may indicate the presence of diseases that affect joint structure and function. It usually involves an initial basic set of tests and may include some follow-up tests depending on the results of the first set of tests, the patient's symptoms, and the disease that the doctor suspects is the cause. Each of these tests can be grouped according to the type of examination that is performed:
How is it used?
Synovial fluid analysis may be ordered to help diagnose the cause of joint inflammation, pain, swelling and fluid accumulation. Diseases and conditions affecting one or more joints and the synovial fluid, can be divided into four main categories:
When is it requested?
Synovial fluid analysis may be requested when a doctor suspects that a patient has a condition or disease involving one or more of their joints. It may be ordered when a patient has some combination of the following signs and symptoms:
It may sometimes be ordered to monitor a patient with a known joint condition.
What does the result mean?
Synovial fluid usually contains a small amount of glucose and protein and may have a few white blood cells (WBCs) and red blood cells (RBCs).
There are a variety of joint abnormalities including osteoarthritis, rheumatoid arthritis, gout, and infection (septic arthritis) that can cause inflammation, swelling, an accumulation of synovial fluid and sometimes bleeding into one or more joints. These conditions can limit mobility and, if left untreated, may permanently damage the joints.
Results of tests performed on a sample of synovial fluid may include:
Physical characteristics - the normal appearance of a sample of synovial fluid is usually:
Changes in the physical characteristics may provide clues to the disease present such as:
Microscopic examination - Normal synovial fluid has small numbers of white blood cells (WBCs) and red blood cells (RBCs) but no microorganisms or crystals present. Laboratories may examine drops of the synovial fluid and/or use a special centrifuge (cytocentrifuge) to concentrate the fluid's cells at the bottom of a test tube. Samples are placed on a slide, treated with special stain, and the different kinds of cells present evaluated.
Finding crystals in the joint fluid and identification of their nature is important in the diagnosis of gout and pseudogout.
Infectious disease tests - in addition to chemistry tests, other tests may be performed to look for microorganisms if infection is suspected.
Is there anything else I should know?
A blood glucose may be requested to compare concentrations with that in the synovial fluid. If a doctor suspects that a patient may have a systemic infection, then a blood culture may be requested in addition to the synovial fluid analysis.
Joint injury, surgery and joint replacement can increase the risk of developing an infection in a joint.
Arthrocentesis is the removal of synovial fluid from a joint with a needle and syringe. A local anaesthetic is applied and then the doctor inserts the needle into the space between the bones and collects the synovial fluid.
Yes. Sometimes it will be performed to drain excess synovial fluid – to relieve pressure in the joint and/or to aid in the treatment of an infection. Sometimes medicines such as corticosteroids will be introduced into the joint to help reduce inflammation and/or to relieve pain.
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