Coagulant proteins and thrombin generation in synovial fluid: A model for extravascular coagulation

The coagulant content and thrombin generating potential of synovial fluid from patients with osteoarthritis were studied as a model of extravascular coagulation. The concentrations of individual coagulant proteins were partially correlated with their molecular weight. The levels of the very large coagulants factor V, factor VIII and von Willebrand factor antigen (vWF:ag) are less than 1% of the activities found in a normal pooled reference plasma while smaller coagulants including factors IX, XI and prothrombin range between 9 and 30%. The protease inhibitors antithrombin‐III (AT‐III) and Alpha‐2 macroglobulin in synovial fluid were present at levels of 74% and 13% of plasma, higher than expected based on their molecular weights. Prothrombin was more rapidly activated by tissue thromboplastin than by aPTT reagent. The thrombin activity formed in synovial fluid decreased more rapidly than that formed in dilute plasma. The addition of recombinant factor VIII or bovine factor V to synovial fluid accelerated the thrombin production by APTT but not by tissue thromboplastin. Indicating that the low levels of factor VIII and factor V did limit the rate of thrombin production. The addition of specific antibodies to factor VIII or factor V strongly inhibited thrombin production by aPTT. These data confirm a roughly inverse relationship between the concentrations of coagulation proteins and their molecular weight in synovial fluid and indicate that thrombin can be generated in synovial fluid. The inactivation of thrombin in synovial fluid may be more dependent on antithrombin‐III than in plasma because of the increased AT‐III/alpha‐2 macroglobulin ratio seen in synovial fluid.