An apparatus for determining blood clotting capacity comprises an actuator to cyclically move a member within a sample of blood received in a well in a tray and one of a deflection sensor and a position sensor to determine the position of the wetted member upon being acted upon by the actuator. The theoretical position of the wetted member, as determined using a known actuator force and wetted member physical data, is compared to the sensed deflection or position of the wetted member, and the resistance to movement of the wetted member caused by the blood is determined and correlated to a clotting capacity.