Apparatus, systems, and methods are provided for assessing clot activity in blood under various conditions in vitro. The apparatus includes a plurality of test receptacles that receive the blood, at least one flow generating mechanism in communication with the test receptacles, and a clot detector configured to analyze an amount of clot formation that occurs within the blood in each receptacle. Blood contacting surfaces of a first subset of the test receptacles are coated with a first surface substrate and blood contacting surfaces of a second subset of the test receptacles are coated with a different second surface substrate. The flow generating mechanism is configured to generate a first blood flow rate through a third subset of the test receptacles and to generate a different second blood flow rate through a fourth subset of the test receptacles.