A novel method for detecting the concentration of a metabolite in a fluid sample is provided. Devices for the detection of the analyte are also provided. In particular, a device for determining the concentration of 11-dehydro thromboxane in a urine sample and comparing it to a set of standardized quartile concentrations is provided. A concentration of urinary 11-dehydro thromboxane that falls within the fourth quartile is indicative of a greatly increased risk of a recurrent cardiovascular event.