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:

• Physical characteristics - an evaluation of the appearance of the fluid
• Chemical tests - detect changes in the chemical constituents of the fluid
• Microscopic examination - cells that may be present are counted and identified by cell type under a microscope. Any crystals present are identified by their shape and their behaviour under polarised light
• Infectious disease tests - detect and identify microorganisms, if present

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:

1. Infectious diseases - those caused by bacteria, fungi or viruses. They may originate in the joint or spread there from other places in the body. These conditions include acute and chronic septic arthritis.
2. Bleeding - bleeding disorders and/or joint injury can lead to blood in the synovial fluid. Commonly present in patients with untreated blood clotting disorders such as haemophilia or von Willebrand disease.
3. Inflammatory diseases

• Conditions that cause crystal formation and accumulation such as gout (needle-like uric acid [monosodium urate] crystals) and pseudogout (calcium pyrophosphate dihydrate crystals). Typically affect the feet and legs.
• Conditions that cause joint inflammation, such as synovitis, or other immune responses. These may include autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus.
  4. Degenerative diseases

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:

• Joint pain
• Redness over the joint
• Joint inflammation and swelling
• Synovial fluid accumulation

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:

• Straw coloured
• Clear
• Moderately viscous - drops of it from a syringe needle will form a “string” a few inches long.

Changes in the physical characteristics may provide clues to the disease present such as:

• Less viscous fluid may be seen with inflammation.
• Cloudy synovial fluid may indicate the presence of microorganisms, white blood cells, or crystals.
• Reddish synovial fluid may indicate the presence of blood, but an increased number of red blood cells may also be present in cloudy synovial fluid.

Chemical test

• Glucose - typically a bit lower than blood glucose levels. May be significantly lower with joint inflammation and infection.

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.

• Total cell counts - number of WBCs and RBCs in the sample. Increased WBCs may be seen with infections and with conditions such as gout and rheumatoid arthritis.
• A WBC differential determines the percentages of different types of WBCs. An increased number of neutrophils may be seen with bacterial infections. Greater than 2% eosinophils may suggest Lyme disease.
• Synovial fluid is evaluated under polarised light to recognise the presence of crystals and to distinguish the types of crystals that are present. Needle-like monosodium urate crystals are associated with gout and calcium pyrophosphate crystals are associated with pseudogout.

Infectious disease tests - in addition to chemistry tests, other tests may be performed to look for microorganisms if infection is suspected.

• Gram stain allows for the direct observation of bacteria or fungi under a microscope. There should be no organisms present in synovial fluid.
• Culture and sensitivity is ordered to determine what type of microorganisms are present. If bacteria are present, antibiotic sensitivity testing can be performed to guide antimicrobial therapy. If there are no microorganisms present, it does not rule out an infection; they may be present in small numbers or their growth may be inhibited because of prior antibiotic therapy.
• Other tests for infectious diseases that are less commonly ordered include AFB smear and culture. This tests for the presence of mycobacteria and may help diagnose tuberculosis. Molecular tests such as PCR for Mycobacteria tuberculosis or other pathogens may also be performed.

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.

Common questions

• What is arthrocentesis and how is it performed?

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.

• Are there other reasons to do an arthrocentesis?

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.

More information

RCPA Manual: Synovial fluid examination

Healthdirect Australia: Gout

Healthdirect Australia: Arthritis