Equine myeloperoxidase: a novel biomarker in synovial fluid for the diagnosis of infection

Reasons for performing study: Equine joint infection is a life-threatening disorder and the confirmation of its diagnosis can be difficult. Synovial fluid biomarkers may assist the discrimination between infectious and non-infectious joint disease. Objectives: The current study investigates whether the immunological detection of total and enzymatically active MPO assists the diagnosis of joint infection in horses. Methods: Four sample groups were included: healthy, OCD, traumatic synovitis and culture confirmed infected joints. Synovial fluid was analyzed for total MPO by a horse specific sandwich ELISA (enzyme-linked immunosorbent assay) and for active MPO using the SIEFED (specific immunologic extraction followed by enzymatic detection) technique. Western blot analysis was performed to confirm the antibody specificity. Results: Samples of infected joints contained significantly more total and active MPO compared to samples from healthy, OCD and traumatic synovitis joints. Cut-off values were set at 5000 and 350 ng/ml for total and active MPO, respectively, with fair sensitivity, specificity, positive and negative predictive values and likelihood ratios for infection. Correlation coefficients between the total as well as the active MPO levels and the routine synovial fluid parameters i.e. the WBC count, the neutrophil count and the total protein level, were reported. No correlation was observed between MPO and either the patients’ age or affected joint. Western blotting confirmed the antibody specificity for equine MPO. Conclusions: Synovial fluid MPO was identified as a very promising biomarker to augment the discrimination of infectious versus non-infectious joint disease in horses. Both ELISA and SIEFED immunoassays can be used for its specific and rapid detection. Potential relevance: The analysis of synovial fluid MPO can be used as a complementary test to aid in the discrimination between infectious and non-infectious joint disease, especially when the white blood cell counts and the total protein level are inconclusive