A method for assessing aspirin resistance and relative risk of a cardiovascular event in a patient taking aspirin is provided. The concentration of 11-dehydro-thromboxane B2 in a urine sample is measured and compared to a set of standardized quartile concentrations. A concentration of urinary 11-dehydro-thromboxane B2 that falls within the second, third, or fourth quartile is indicative of aspirin resistance and an elevated risk of a recurrent cardiovascular event.