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.