- The major components of synovial fluid analysis are assessing fluid clarity and color, determining the cell count, examining for crystals, and obtaining culture.
- Joint aspiration should be performed promptly whenever septic arthritis is suspected because synovial fluid cell count, Gram stain, and culture are necessary to establish or exclude joint space infection.
- Synovial fluid analysis can be diagnostic in cases of crystalline arthritis.
- The synovial fluid white cell count is the most reliable means of distinguishing noninflammatory (<2000 cells/mcL) from inflammatory (>2000 cells/mcL) forms of arthritis.
- Joint injections with glucocorticoid are often the swiftest means of providing relief to patients with inflamed joints.
Joint aspiration with subsequent synovial fluid analysis, Gram stain, and culture should be performed promptly whenever there is clinical suspicion of septic arthritis (eg, unexplained, acute monoarticular arthritis). The presence of crystals in synovial fluid can be diagnostic of gout as well as pseudogout and calcium pyrophosphate dehydrate deposition disease (CPPD). The synovial fluid white cell count is the most reliable means of distinguishing noninflammatory from inflammatory forms of arthritis. As a general guide, synovial fluid should be examined when the underlying cause of arthritis is uncertain and arthrocentesis is feasible.
The major components of synovial fluid analysis are (1) assessing fluid clarity and color, (2) determining the cell count, (3) examining for crystals, and (4) obtaining cultures. When septic arthritis is suspected, a Gram stain should also be performed. Determinations of synovial fluid glucose and protein have little diagnostic value and should not be ordered. Although the viscosity of synovial fluid decreases with inflammation, evaluations of viscosity are not standardized and add little to the diagnostic value of synovial fluid analysis.
Examination of the synovial fluid begins with a visual determination of clarity and color. Although crystals, lipids, and even cellular debris may affect clarity, the major determinant of synovial fluid clarity and color is the cell count. Noninflammatory fluid, such as that associated with osteoarthritis, has a low cell count and is clear. Synovial fluid from moderately inflammatory forms of arthritis, such as systemic lupus erythematosus or mild rheumatoid arthritis, has higher cell counts and is translucent and yellow. Fluid from intensely inflammatory processes, such as septic joints or crystal-induced arthropathies, has very high cell counts and is opaque and white to yellow. Bleeding into a joint leads to a hemarthrosis with characteristic opaque, red synovial fluid.
Normal synovial fluid has <200 white cells/mcL, most of which are mononuclear. In pathologic effusions, the synovial fluid white cell count discriminates between noninflammatory forms of arthritis (<2000 white cells/mcL) and inflammatory arthritis (>2000 white cells/mcL with a neutrophil predominance). The synovial fluid white cell count can be an approximate guide to the cause of the underlying inflammatory arthritis (see below, “Classes of Synovial Fluid”).
Crystal analysis is best performed on ...