A method of determining a contact activator for platelet activation and/or clotting activation is provided, comprising placing a predetermined amount of an anticoagulant in each cell of a multicell test cartridge, placing a predetermined amount of a platelet inactivating agent in each cell, and placing a measured amount of contact activator in each cell, the amount of contact activator in each cell differing from the amount in each other cell. An aliquot of a blood sample is added to each cell, and the blood sample aliquot, clotting reagent and platelet inactivating agent are mixed. Each cell sample is allowed to clot, and the clotting time for each cell is measured. The relative clotting times are used to determine the platelet activation and/or clotting activation effect of the clotting activator.