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 blood in the synovial fluid. Commonly present in patients with untreated blood clotting disorders such as haemophilia or von Willebrand Disease.

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 these 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.

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 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 tests – tests that may be performed on synovial fluid samples may include:

• Glucose—typically slightly lower than blood glucose levels. May be significantly lower with joint inflammation and infection.
• Protein—increased with bacterial infection
• Lactate dehydrogenase (LD)—increased in rheumatoid arthritis, infectious arthritis, or gout
• Uric acid—increased in gout

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.

• 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 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 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 requested include AFB smear and culture. This tests for the presence of mycobacteria and may help diagnose tuberculosis. Molecular tests for Mycobacteria tuberculosis which are more sensitive and specific may also be performed.

   

A blood or urine uric acid or blood glucose may be requested to compare concentrations with those 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.