Osteoarthritis is a dynamic disease process characterized by varying degrees of synovial inflammation and fibrosis, depending on the stage of disease. Inflammation of the synovial membrane predominates in early osteoarthritis, while fibrosis ensues later in disease. Identification of biomarkers associated with the early, inflammatory phases of osteoarthritis will enable clinicians to stage disease and determine appropriate therapeutic timing. Alpha 2- macroglobulin (A2M) is a novel intra-articular therapy for equine joint disease, and proteoglycan 4 (PRG4) is under development as a potential osteoarthritis therapeutic; however, it is still unknown which patients are likely to benefit most from these therapeutic interventions. Synovial fluid proteomics pilot data from our lab identified several proteins most upregulated in equine carpal osteoarthritis, including PRG4, inter-α-trypsin inhibitor (IαI), and A2M. Interestingly, many highly upregulated proteins have primary or secondary immunomodulatory roles. We hypothesize that a panel of synovial fluid protein biomarkers can aid in both staging equine osteoarthritis and guiding therapy. Here, we will validate synovial fluid proteomics data with PRG4, IαI, and A2M gene expression and immunoassays and correlate these biomarkers with the degree of inflammation and fibrosis using synovial membrane histology and synovial fluid PGE2. This study will lay the groundwork for understanding how these synovial fluid proteins are regulated throughout the course of osteoarthritis and provide context for the appropriate case selection and timing of administration of emerging therapeutics such as A2M and PRG4.