proteinases, and collagenases. Synovial fluid exhibits non-Newtonian flow characteristics; the viscosity coefficient is not a constant and the fluid is not linearly viscous. Synovial fluid has thixotropic characteristics; under conditions of sudden increase in pressure as in some kind of mechanical shock, its viscosity suddenly increases. Normal synovial fluid contains 3–4 mg/ml hyaluronan (hyaluronic acid), a polymer of disaccharides composed of D-glucuronic acid and D-N-acetylglucosamine joined by alternating beta-1,4 and beta-1,3 glycosidic bonds. Hyaluronan is synthesized by the synovial membrane and secreted into the joint cavity to increase the viscosity and elasticity of articular cartilages and to lubricate the surfaces between synovium and cartilage. Synovial fluid contains lubricin (also known as PRG4) as a second lubricating component, secreted by synovial fibroblasts. Chiefly, it is responsible for so-called boundary-layer lubrication, which reduces friction between opposing surfaces of cartilage. There also is some evidence that it helps regulate synovial cell growth. It also contains phagocytic cells that remove microbes and the debris that results from normal wear and tear in the joint. Clinical significance Collection Synovial fluid may be collected by syringe in a procedure termed arthrocentesis, also known as joint aspiration. Classification Synovial fluid may be classified into normal, noninflammatory, inflammatory, septic, and hemorrhagic: Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum. Synovial fluid viscosity Normal: Normal Traumatic arthritis Degenerative (Osteo) arthritis Pigmented villonodular synovitis Normal or decreased: Systemic lupus erythematosus Decreased: Rheumatic fever Rheumatoid arthritis Gout Pyogenic (Septic) arthritis Tubercular arthritis Less of lubrication in joints Pathology Many synovial fluid types are associated with specific diagnoses: Analysis Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum. The cytological and biochemical analysis of human synovial fluid began around 1940 using cadaver-derived fluid and comparing characteristics to those of, for instance, bovine synovial fluid. Chemistry The mucin clot test is a very old approach to determining if an inflammatory infiltrate is present. In this test, acetic acid is added to the synovial fluid specimen. In a normal specimen, this should lead to a congealing of the hyaluronic acid, forming a 'mucin clot.' If inflammation is present, a mucin clot is not formed (the hyaluronic acid is degraded). Lactate is elevated in septic arthritis, usually above 250 mg/dL. Complement factors are decreased in rheumatoid arthritis and lupus arthritis. Microscopy Microscopic analysis of synovial fluid is performed to evaluate for cell count and crystals. Crystals include monosodium urate crystals,