Synovial Fluid Analysis
When one or more of your joints are swollen, red, and/or painful
A sample of synovial fluid is collected by a doctor from the affected joint with a syringe and needle using a procedure called a joint aspiration. The sample is called a joint aspirate.
Consult with your doctor about test preparation. Synovial fluid collection and analysis may be performed after fasting or at random.
Synovial fluid is a thick liquid that acts as a lubricant for the major joints of the body. It is found in small quantities in the spaces between the bones in a joint, called the synovial cavity. Here the fluid is produced and contained by the synovial membranes which cover the inner surface of the cavity. Synovial fluid lubricates the joint and reduces friction during movement in the knees, shoulders, hips, hands, and feet.
Synovial fluid analysis consists of inspection of fluid, as well as tests that detect changes in synovial fluid that may indicate the presence of diseases that affect joint structure and function.
How is it used?
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.
When is it requested?
- 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.
What does the test result mean?
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
- 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.
- 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.
Is there anything else I should know?
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.
What is arthrocentesis and how is it performed?
Are there other reasons to do an arthrocentesis?
On This Site
Tests: Gram stain; White blood cell count (WBC); Red blood cell count (RBC); C reactive protein (CRP); erythrocyte sedimentation rate (ESR); blood culture; uric acid test
Conditions: Arthritis, Gout, Rheumatoid arthritis, Septic arthritis, Tuberculosis
Elsewhere On The Web
UK Gout Society
Patient: Septic Arthritis
MedlinePlus Medical Encyclopedia: Synovial fluid
JAMA Patient Page: Septic Arthritis
BSR & BHPR, BOA, RCGP and BSAC guidelines for management of the hot swollen joint in adults