Diagnostic methods for determining whether an individual will benefit from a particular anti-thrombotic therapeutic agent are disclosed. The methods involve obtaining a biological sample that comprises platelets, from a patient who has been pre-administered a particular therapeutic agent, which is an antagonist of a receptor associated with the biochemical pathways involved in platelet aggregation, and exposing the platelets to an agonist of the receptor. If the antagonist is ineffective, the platelets will eject microparticles, will have a different size distribution than platelets not exposed to the agonist, and will experience a change in their surface charge. In one embodiment, the diagnostic methods involve using single particle optical sizing techniques to determine the presence of such ejected microparticles, or a change in platelet size due to its activation by the agonist. In another embodiment, electrophoretic quasi-elastic light scattering techniques are used to determine the presence of a change in surface charge on the platelets. Once an effective therapeutic agent, or an effective dosage of such therapeutic agent, has been identified, the patient can begin therapy knowing that the agent will be effective.La présente invention concerne des procédés de diagnostic permettant de déterminer si un sujet va bénéficier dun agent thérapeutique anti-thrombotique particulier. Les procédés comprennent lobtention dun échantillon biologique comprenant des plaquettes, provenant dun patient auquel un agent thérapeutique particulier a été préalablement administré, qui est un antagoniste dun récepteur associé aux voies biochimiques impliquées dans lagrégation plaquettaire, et lexposition des plaquettes à un agoniste du récepteur. Si lantagoniste est inefficace, les plaquettes vont éjecter des microparticules, vont avoir une distribution de tailles différente de celle des plaquettes non exposées à lagoniste, et vont subir un changement dans leur charge superficielle. Sel
