Synovial Fluid Analysis

Synovial fluid analysis may be requested 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:

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.

Bleeding: bleeding disorders and/or joint injury can lead to bleeding in the synovial cavity. Commonly present in patients with untreated blood clotting disorders such as haemophilia or von Willebrand Disease.

Inflammatory diseases including:

Conditions that cause crystal formation and accumulation such as gout (needle-like uric acid crystals) and pseudogout (calcium pyrophosphate dihydrate crystals). Typically these affect the feet and legs. The diagnosis of gout is usually supplemented by measuring uric acid in blood.

Conditions that cause immune mediated joint inflammation including autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus (SLE). Other disorders such as sarcoidosis, Crohn’s disease and ulcerative colitis may be associated with arthritis. Infections in other parts of the body may produce a “reactive” arthritis also known as Reiter’s syndrome.

Degenerative diseases such as osteoarthritis.

Synovial fluid analysis may be requested when a doctor suspects that a patient has a condition involving one or more of their joints and some of the following signs and symptoms:

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

It is sometimes requested to monitor a patient with a known joint condition.

Synovial fluid aspirates usually contain a small amount of glucose and protein and may have a few white blood cells (WBCs) and red blood cells (RBCs).

There are a wide 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.

The normal appearance of a sample of synovial fluid is usually:

• Straw coloured
• Clear
• Viscous (thick) – 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 in the presence of 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.

The tests most commonly performed on synovial fluid are to look for microorganisms when infection is suspected. These include:

• Gram stain allows the direct observation of bacteria or fungi under a microscope. There should be no organisms present in synovial fluid.
• Culture and sensitivity is used to determine what type of microorganism is 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.

In addition:

• Synovial fluid may be evaluated under polarised light to identify the presence of crystals and to distinguish the types of crystals, if present. Long needle-like monosodium urate crystals are associated with gout and short broader calcium pyrophosphate crystals are associated with pseudogout.

Other laboratory investigations which may be requested if a doctor suspects that a patient may have a systemic infection, include a full blood count (FBC), C reactive protein (CRP), and erythrocyte sedimentation rate (ESR). If gout is suspected, the doctor will request serum uric acid or urate. Some infections such as Lyme Disease (Borreliosis) are not easily diagnosed by joint aspirate and may require a specific blood test.

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.