Flow cytometric detection of circulating platelet-derived microparticles in healthy adult horses.

OBJECTIVE To develop a flow cytometric assay to quantify platelet-derived microparticles (PMPs) in equine whole blood and plasma. SAMPLE Citrate-anticoagulated whole blood from 30 healthy adult horses. PROCEDURES Platelet-poor plasma (PPP) was prepared from fresh whole blood by sequential low-speed centrifugation (twice at 2,500 × g). Samples of fresh whole blood and PPP were removed and stored at 4° and 24°C for 24 hours. Platelet-derived microparticles were characterized in fresh and stored samples on the basis of the forward scatter threshold (log forward scatter < 10(1)) and labeling with annexin V (indicating externalized phosphatidylserine) and CD61 (a constitutive platelet receptor). A fluorescent bead-calibrated flow cytometric assay was used to determine microparticle counts. Platelet counts, prothrombin time, and activated partial thromboplastin time were measured in fresh samples. RESULTS Significantly more PMPs were detected in fresh whole blood (median, 3,062 PMPs/μL; range, 954 to 13,531 PMPs/μL) than in fresh PPP (median, 247 PMPs/μL; range, 104 to 918 PMPs/μL). Storage at either temperature had no significant effect on PMP counts for whole blood or PPP. No significant correlation was observed between PMP counts and platelet counts in fresh whole blood or PPP or between PMP counts and clotting times in fresh PPP. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that the described PMP protocol can be readily used to quantify PMPs in equine blood and plasma via flow cytometry. Quantification can be performed in fresh PPP or whole blood or samples stored refrigerated or at room temperature for 24 hours.