Platelet rich plasma (PRP) is a popular autologous biological therapy used for the treatment of various equine ailments, including tendon and ligament injuries, osteoarthritis, and cutaneous wounds. A number of commercial products are available for producing PRP, each generating a slightly different product. Variations in platelet numbers and white blood cell (WBC) counts are believed to be the most critical variables, as they are directly related to concentrations of growth factors and inflammatory cytokines. Accurate documentation of platelet numbers is essential for prospective evaluation of clinical outcomes, but can be problematic in platelet concentrates depending on the counting method employed. The objectives of this study were to compare the performance of four platelet counting technologies in equine platelet concentrates and to evaluate the ability of the Magellan PRP system to concentrate equine platelets. We hypothesized that there would be no differences in platelet counts among the four counting technologies and that the Magellan system would generate platelet concentrations greater than 500,000/μL. Citrated whole blood was collected from 32 horses and platelet, WBC, and red blood cell concentrations were measured using a commercial hematology analyzer (Advia 2120) prior to preparation of PRP using the Magellan system. Platelets were quantified in individual identical aliquots of equine PRP produced by the Magellan system (n=32) using three different technologies: optical scatter (Advia 2120), impedance (CellDyn 3700), and hand count using direct microscopy (Thrombo-TIC). An immunofluorescent counting method was performed on a subset of 15 of the 32 samples using a mouse monoclonal anti-sheep antibody against integrin alpha αIIbβ₃ (anti-CD41/CD61) and a fluorescent secondary antibody. Measured platelet concentrations were compared using Passing and Bablok regression analyses and mixed model ANOVA. The Magellan PRP system yielded mean (± SD) platelet and WBC counts of 893,090 ± 226,610/μL and 35,806 ± 9,971/μL, respectively. Platelet counts generated by optical scatter were consistently higher than those generated by impedance. Systematic and proportional biases were observed between these two automated methods. No bias (systematic or proportional) was observed among any of the other counting methods. Despite the bias detected between the two automated systems, there were no significant differences on average among the four counting methods evaluated, based on the ANOVA. All four platelet counting methods tested are therefore suitable for quantifying platelets in equine PRP for clinical applications. The Magellan PRP system consistently generated desirably high platelet concentrations as well as higher than expected WBC concentrations. The high platelet concentrations served as a good test medium for the study; however, the concurrent high WBC counts may be undesirable for selected orthopedic applications. / Master of Science