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The synovial fluid of 112 dogs, patients at the Small Animal Clinic of the Free University of Berlin, was analyzed in this study. Synovial fluid was aspirated from 130 individual joints; all 130 samples were analyzed.Eighteen control samples taken from 8 healthy dogs served as unaffected controls. The following diseases where represented in the osteoarthritis group: 56 dogs (56 joints) suffered from osteoarthritis secondary to cranial cruciate ligament rupture, 16 dogs (24 sample) had fragmented medial coronoid processes, 11 dogs (11 samples) had dysplastic hips, 11 dogs (11 samples) had osteochondrosis dissecans and 10 dogs (10 samples) suffered from miscellaneous joint traumas.Attention was given to total nucleated cell count, viscosity, specific gravity, total protein, and macroscopic appearance during analysis. Special attention was given to myeloperoxidase content.Myeloperoxidase catalyzes the formation of a powerful oxidant, hypochlorus acid (HOCI), from hydrogen peroxide and chloride anions. Hypochlorus acid has the ability to disrupt large chains of hyaluronic acid, forming smaller ogliometric units. Articular cartilage is cut at the ß-acetyl groups of the matrix components. Radiographic and intra-operative examinations were performed on all joints aspirated and served to document the joint state.The goal of this study was to examine the role of myeloperoxidase as a marker for the diagnosis of osteoarthritis. In the course of this study myeloperoxidase levels were compared to the synovial fluid analysis of the control group and radiographic and macroscopic studies.The results of myeloperoxidase measurements in the synovial fluid demonstrated a highermyeloperoxidase content in all joints of the affected group than in the joints of the controlgroup. The groups showed no overlapping values. The minimum of the affected group wastwo times higher than in the maximum of the control group.The means of the control and the affected groups differ greatly. The affected group with the lowest mean myeloperoxidase content (chronic cranial cruciate ligament rupture) had a concentration of 0.72 U/200yl which was 24 times higher than that of the control group (0.029 U/200yl).From these results it is easily recognizable that myeloperoxidase in synovial fluid is a valuable marker for the identification of an osteoarthritic joint. The comparison of myeloperoxidase concentration with total cell count radiographic and macroscopic analysis showed no correlation. However, viscosity correlated with myeloperoxidase content. This result was expected because hypochlorus acid (a myeloperoxidase product) breaks down large chain hyaluronic acid into smaller units and lowers synovial fluid viscosity.Review of these results shows the potential of myeloperoxidase determination as a diagnostic aid, especially in the diagnosis of cases of fragmented coronoid processes and osteochondrosis dissecans. Other diagnostics such as synovial fluid analysis and radiographic studies could not confirm the diagnosis until surgical intervention. In contrast, myeloperoxidase evaluation was definitive. In suspected cases of degenerative joint disease, myeloperoxidase determination is an excellent test for confirming a diagnosis and useful in deciding to intervene either surgically or endoscopically.The results of this study demonstrate the usefulness of myeloperoxidase as a diagnostic tool in cases of osteoarthritis; the potential of monitoring osteoarthritis progression and aiding in the evaluation of the efficacy of new drugs deserves further research.