The performance of platelets can be best understood by monitoring both the force development and the elastic modulus of the blood clot during both clotting and dissolution of the blood clot. Intermittent application and removal of a compressive force on a blood sample (8) positioned between a pair of plates (12 and 14) throughout clot formation and dissolution provides a standard for using voltage output from a transducer (22) to determine force and elastic modulus parameters. Force development arises from the internal actions of the platelets during clot retraction. Elastic modulus provides a measure of the stiffness of the clot. Clotting can be measured as an increase in force development and clot elastic modulus. Clot dissolution can be determined by a dramatic decrease in force development and in elastic modulus. Providing the blood sample with a clot dissolving agent such as tPA allows determining the fibrinolytic potential of whole blood and, thus, aids in identifying patients at risk for thrombosis due to a decreased ability to dissolve clots. Clot elastic modulus provides a measure of erythrocyte flexibility.