Substantially more gelatinases appear in effusions of septic arthritis than in effusions of aseptic arthritis. We hypothesized there is greater plasminogen activator/plasmin activity in effusions of septic arthritis than aseptic arthritis. We examined the antigenic values of urokinase-type plasminogen activator and plasminogen activator inhibitor Type-1, cell counts, and levels of matrix metalloproteinase-2 and metalloproteinase-9 in 135 knee effusions from 80 patients with septic arthritis, rheumatoid arthritis, gouty arthritis, and osteoarthritis. Urokinase-type plasminogen activator and plasminogen activator inhibitor Type-1 antigenic values in effusions of septic arthritis were greater than those in effusions of aseptic arthritis. The increases of urokinase-type plasminogen activator and plasminogen activator inhibitor Type-1 antigenic values in effusions were associated with increased levels of prometalloproteinase-9 and the appearance of activated metalloproteinase-2. Antigenic values of urokinase-type plasminogen activator also correlated with the appearance of activated metalloproteinase-9. High plasminogen activator/plasmin activity, prometalloproteinase-9 levels, and the presence of activated metalloproteinase-2 and metalloproteinase-9 in effusions from replaced knees should increase suspicion of infection regardless of neutrophil counts. Joint aspiration reduces bacteria counts, endotoxins, proinflammatory cytokines, and matrix metalloproteinase. It also decreases the plasminogen activator/plasmin activity in effusions that may play a part in extracellular matrix destruction.
Level of evidence: Diagnostic study, Level III (study of nonconsecutive patients without consistently applied reference "gold" standard). See the Guidelines for Authors for a complete description of levels of evidence.