Aspiration and Injection Techniques of the Lower Extremity

Christopher P. Chiodo, MD; Catherine Logan, MD, MBA, MSPT; Cheri A. Blauwet, MD


J Am Acad Orthop Surg. 2018;26(15):e313-e320. 

In This Article

Synovial Fluid Analysis

Joint fluid analysis is a useful diagnostic tool in the management of both septic arthritis and inflammatory disease. Analysis of the aspirate is critical in the treatment of adult septic arthritis because it guides antibiotic management. It also allows for the establishment of an accurate diagnosis in crystalline disease and guides management of inflammatory arthritis. The macroscopic appearance of synovial fluid provides immediate information to apply toward the differential diagnosis. The color, clarity, and viscosity can be appreciated on gross examination (Table 3). Synovial fluid specimens are placed in specimen containers specific to the test being ordered. A heparinized tube is preferable for cell counts, sterile containers for microbiology testing, and plain tubes for chemistry and immunological testing of the fluid. The volume aspirated from the joint may be small, and if only a few milliliters of fluid are available, preference should be given for cell count analysis. Normal synovial fluid is straw-colored, clear, and viscous. Increased inflammation changes the fluid's macroscopic appearance; the color ranges from yellow to greenish yellow, the clarity is more opalescent, and the viscosity is decreased. In the setting of a pyogenic infection, the aspirate may appear as frank, purulent material. Bloody fluid may be the result of joint trauma, coexisting anticoagulation therapy, baseline coagulation disorders, or synovial tumors such as pigmented villonodular synovitis.[3] The appearance of particles other than cells may indicate the presence of crystals. In rare instances, concomitant septic and gouty arthritis exists;[2] therefore, early diagnosis requires a high level of suspicion because there may be an absence of fever or leukocytosis. Synovial fluid aspirate should be sent for cell count and differential, Gram stain, microbiologic culture, and crystal analysis,[1] with results guiding subsequent management (Table 3). Of note, the cell count suggestive of septic arthritis in a prosthetic joint is much lower compared with a native joint. Additional biochemical studies, including the analysis of glucose, protein, and complement level, may be performed to provide further characterization of inflammatory arthritides.