Arthrocentesis, with subsequent synovial fluid analysis, is used to determine the cause of arthritis in all cases presenting with joint effusion or inflammatory signs. This most commonly occurs in the evaluation of septic arthritis (gram stain, culture and sensitivity) or suspected crystal-induced arthritis (to differentiate gout from pseudogout).
The synovial fluid is examined for physical characteristics (color, viscosity), chemistry (glucose, protein, uric acid) and microscopy (white blood cell counts, differential count, gram stain).
A normal sample of synovial fluid is usually straw colored and clear.
- A cloudy sample may indicate the presence of bacteria, white blood cells or crystals
- A reddish sample may indicate the presence of blood
It is moderately viscous and as a result will form a string longer than 3 cm if a drop is expressed into a test tube (string test). The test depends on the polymerization of hyaluronic acid.
- A less viscous fluid is seen with inflammation. The string test is negative (shorter than 3 cm) due to dilution of hyaluronic acid by entry of extracellular fluid into the inflamed joint
Chemical tests that are performed on synovial fluid include glucose, protein and uric acid:
- Glucose is significantly lower with joint inflammation and infection
- Protein levels are increased in bacterial infection due to leakiness of the synovial membrane that results from inflammation. This leads to an increased concentration of plasma proteins
- Uric acid is increased with gout
Inflammation leads to an influx of white blood cells into the synovial tissue. Microscopic examination is used to detect white blood cell counts, differential counts and red blood cells. These results help differentiate the most common causes of joint effusion into septic effusions, inflammatory infusions, non-inflammatory (degenerative diseases such as osteoarthritis) effusions and hemorrhagic effusions.
1. Septic effusions
Septic effusions may be due to bacteria, fungus or TB. They are characterized by:
- Physical appearance: joint fluid is opaque
- White blood cell (WBC) counts above ≥2000/mm3 and more frequently above 50,000/mm3
- Neutrophil count is usually above 75% in bacterial infection
- A high lymphocyte count is associated with mycobacterial (TB) infection
- An eosinophil count above 2% may suggest Lyme disease
2. Inflammatory effusions
Inflammatory effusions include rheumatoid arthritis, gout, pseudogout and may overlap with the picture of septic arthritis explained above. They are characterized by:
- Physical appearance: joint fluid can be translucent/opaque
- WBC counts above ≥2000/mm3 but not as high as septic arthritis
- Neutrophil count is usually in the range of 50%
- Negative Gram stain and bacterial cultures
- Polarizing microscopy shows needle-like monosodium urate (MSU) crystals in gout or calcium pyrophosphate (CPPD) crystals in pseudogout. In the case of gout crystals seen under a polarizing microscope are birefringent, meaning they exhibit (1) yellow needle like crystals when parallel to north-south polarizing axis and (2) blue needle like crystals when oriented east-west in relation to polarizing axis.
3. Non-inflammatory (degenerative) effusions
Non-inflammatory effusions are caused by degenerative conditions such as osteoarthritis, avascular necrosis, or meniscal tear. They are characterized by:
- Physical appearance: joint fluid is clear and transparent
- WBC <2000/uL/mm3
- Neutrophil < 25%
- Osteoarthritis, avascular necrosis and meniscal tear are differentiated clinically.
4. Hemorrhagic effusions
Hemorrhagic effusions are caused by trauma, bleeding disorder such as hemophilia, anticoagulation, neuropathic arthropathy or neoplasms (e.g. synovioma, hemangioma as well as malignant tumors). They are characterized by:
- Physical appearance: joint fluid is hemorrhagic
- Elevated red blood cell counts
The clinical picture further helps to distinguish the various disease states such as rheumatoid arthritis, pseudogout or septic arthritis.