The invention provides a method of monitoring platelet function in a mammal by passing blood removed from the body of the mammal through a passageway to contact an obstruction or irregularity in the passageway to generate a platelet mass in the passageway, and monitoring the flow or composition of the blood in the passageway to detect the platelet mass. The flow and composition change in response to the formation of a platelet mass in the passageway. Devices, articles, and kits for performing the methods are also disclosed.