What are the typical features of the synovial fluid in Rheumatoid Arthritis?
Rheumatoid arthritis (RA) is an autoimmune disorder that primarily affects the joints, leading to inflammation, pain, and damage. The synovial fluid is a viscous fluid that lubricates and nourishes the joints, and its characteristics can provide insights into the underlying condition. In RA, the synovial fluid often shows specific features indicative of the inflammatory process. Some typical features of synovial fluid in rheumatoid arthritis include:
- Increased Viscosity: Synovial fluid in RA tends to be more viscous or thickened compared to normal synovial fluid. This is due to the increased presence of inflammatory cells and proteins.
- Increased Volume: Inflammation in RA leads to synovial membrane thickening and increased production of synovial fluid. This can cause joint swelling and an increase in the amount of synovial fluid present.
- Cloudy Appearance: The synovial fluid may appear cloudy or turbid due to the presence of inflammatory cells, immune cells, and proteins.
- Elevated White Blood Cell Count: The number of white blood cells (leukocytes) in the synovial fluid is typically increased in RA. These cells, particularly neutrophils, are involved in the inflammatory response.
- Presence of Inflammatory Markers: Inflammatory markers such as C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) may be elevated in the synovial fluid, reflecting the ongoing inflammation in the joints.
- Increased Protein Content: RA synovial fluid often contains higher levels of proteins, including immunoglobulins and other antibodies, which contribute to the immune response.
- Presence of Rheumatoid Factor (RF) and Anti-Citrullinated Protein Antibodies (ACPAs): These autoantibodies are commonly found in the synovial fluid of individuals with RA. They play a role in the immune response and contribute to the joint damage seen in the disease.
- Decreased Viscosity: In later stages of RA, the synovial fluid might lose some of its normal viscosity due to the degradation of cartilage and joint tissues.
The synovial fluid is inflammatory, with white blood cell (WBC) counts typically between 5000 and 50,000/mm 3 .
Rarely, synovial fluid WBC count can exceed 100,000/mm 3 (pseudoseptic) but infection must always be ruled out. Generally the differential shows a predominance (>50%) of PMNs.
The protein level is elevated, and the glucose level may be lower than serum values (40%–60% of serum glucose). There are no crystals in the fluid, and cultures are negative.
Unfortunately, there are no specific findings in the synovial fluid that allow a definitive diagnosis of RA.
