Synovial fluid analysis is often done to aid in the diagnosis of joint inflammation, pain, swelling, and/or fluid accumulation. It is withdrawn from the joint by a procedure called arthrocentesis.
What is Synovial Fluid?
The synovial fluid or joint fluid is present in all joints or diarthroses. The articular surfaces of the synovial joints are covered by articular cartilage and are separated from each other by a small cavity that contains the synovial fluid.
The synovial joint has a joint capsule which contains specialized secretory cells termed synoviocytes (types A and B). These produce the components of the synovial fluid and contribute to the local production of cytokines, small-molecule mediators of inflammation, and proteolytic enzymes.
Synovial fluid also provides the nutrients and lubrication for the joints.
Biochemically, the synovial fluid is an ultrafiltrate of plasma.
In normal physiological conditions, the biochemical composition of the synovial fluid is similar to that of plasma.
This composition changes in various diseases and results in gross, microscopic, biochemical and cytological changes in the fluid. These changes can be analyzed and information can be deduced from the values generated in the analysis.
Typical characteristics of synovial fluid
- Appearance – clear
- Viscosity – high
- Moderately viscous – drops of it from a syringe needle will form a “string” a few inches long.
- A small amount of glucose and protein.
- Glucose level is similar to that of the patient’s serum glucose level
- Few white blood cells (WBCs< 200/ µL) and red blood cells
- No microbes or crystals present
- Polymorphonuclear neutrophils (PMNs< 25% )
Pathologies Affecting Joints – Indications for Synovial Fluid Analysis
Diseases and conditions affecting joint(s) and the synovial fluid can be divided into four main categories:
These could be caused by bacteria, fungi, or viruses. Infection of the joint results in acute and chronic septic arthritis.
Bleeding disorders and/or joint injury can lead to blood in the synovial fluid
- Conditions that cause crystal formation and accumulation such as gout and pseudogout
- Conditions that cause joint inflammation, such as synovitis, or other immune responses. These may include autoimmune disorders such as rheumatoid arthritis and lupus.
Collection of Synovial Fluid
Synovial fluid is collected via arthrocentesis or joint aspiration. The collected volume depends on the size of the joint and the necessary tests. Normal joint contains few ml of joint fluid but this quantity increases in the presence of inflammation.
Depending on the test, different types of preservatives are required, as follows:
- Sterile heparinized tubes for microbiology testing
- Heparinized or EDTA tubes for cell count
- Without preservatives for most chemistry tests
- NaF for glucose testing
Powder preservatives or powdered gloves should not be used for sample collection as they can introduce particulates that could interfere with the synovial crystal analysis
Synovial Fluid Analysis – Abnormalities and Interpretation
Lab evaluation of synovial fluid involves the following three steps
- Physical examination like appearance, color, viscosity etc
- Chemical analysis like glucose, total protein level, uric acid etc
- Microscopic evaluation like cell count and differential count, crystal identification etc
Additionally, microbiologic, serologic, and cytologic laboratory evaluations can be done.
Changes in the appearance of synovial fluid
Changes in the physical characteristics may provide clues to the disease present, such as:
- Decreased viscosity may be seen with inflammation. Viscosity decreases because of decreased production and polymerization of hyaluronic acid. A string 4-6 cm of synovial fluid between the tips of the fingers is normal.
- Cloudy synovial fluid may indicate the presence of microbes, white blood cells, or crystals.
- Reddish synovial fluid may indicate the presence of blood.
- Inflammatory diseases of nonimmunologic origin: Cloudy/turbid, dense yellow with crystals
- High number of crystals are present: Milky/cloudy
Changes in the biochemical composition of synovial fluid
In normal circumstances, the concentration of most of the analytes of synovial fluid is the same as that in serum.
For accurate interpretation of synovial fluid laboratory results, serum samples should be concomitantly evaluated.
Glucose is significantly decreased in inflammatory conditions. These include
- Immunologic disorders
- Crystal-induced arthropathies [gout and pseudogout]
- Rheumatoid arthritis
- Septic arthritis
Samples used for glucose evaluation should be collected in NaF [sodium fluoride] tubes to prevent rapid glycolysis and should be tested within 1 hour after collection. Otherwise, a falsely decreased glucose concentration is obtained.
Total protein concentration in synovial fluid is lower than in serum, usually one-third of that in serum. However, this concentration increases in inflammatory and hemorrhagic disorders.
Presence of increased uric acid indicates gout and confirmation is done by a high number of uric acid crystals in synovial fluid.
Cell counts in synovial fluid analysis
Increased WBC count [normal 200 cells/µL] indicates infections/inflammation. Gram stain and culture can be used for differential diagnosis.
- Total cell counts— the number of WBCs and RBCs in the sample Increased WBCs are seen in infection and in conditions such as gout and rheumatoid arthritis.
- Increased number of neutrophils – bacterial infections.
- > 2% eosinophils may suggest Lyme disease, parasitic arthritis, rheumatoid disease, or tubercular arthritis.
Microscopic evaluation of crystals in synovial fluid
- Monosodium urate
- In gout
- Crystals needle-shaped
- Negative/yellow birefringence in the compensated polarized light.
- Calcium pyrophosphate (CPP) crystals
- Positive/blue birefringence in the compensated polarized light.
- Cholesterol crystals
- In hyperlipoproteinemia and rheumatoid arthritis
- Rhombic and have negative/yellow birefringence in the compensated polarized light.
- Corticosteroids crystals
- Seen in patients treated with intra-articular injections of corticosteroids.
- Flat, small, and have different shapes
- Can have both positive/blue and negative/yellow birefringence
- Calcium phosphate crystals [apatite]
- Degeneration and calcification of cartilage and osteoarthritis.
- Coffin-lid shaped
- No birefringence
- Calcium oxalate crystals
- In renal diseases or dialysis.
- Envelopes or pyramid shaped
- Negative/yellow birefringence
Further Lab Studies for Infection
When infection is suspected, the synovial fluid may be subjected to special investigations. These are
This involves staining the smear of the fluid on the slide with Gram’s stain and direct observation of bacteria or fungi under a microscope.
Normally, there should be no microbes present in synovial fluid.
Culture and Sensitivity
To determine the type of microbe and its susceptibility to different drugs.
AFB Staining and Culture
To investigate the presence of tuberculosis. Molecular tests like PCR are also methods for Mycobacteria tuberculosis detection.