Methods, compositions and kits are disclosed for identifying patients with an increased risk of experiencing an adverse cardiovascular event where the patient is undergoing aspirin antiplatelet therapy. A platelet-containing sample from the patient is evaluated for platelet hyperactivity and platelet hyperactivity in the sample is related to the patient's risk of an adverse cardiovascular event. In some embodiments the evaluation for platelet hyperactivity is carried out by assessing the function of platelets in the sample using a high shear platelet function test.