Principle of the GTT. A: Blood flows at 37°C under the influence of gravity through a narrow gap  formed between the larger ball bearing and the inner wall of the tube, where high shear stress (175 dyn/cm2) activates platelets. These activated platelets remain single, since the very short transit time and high shear prevent aggregation. In contrast, in the space downstream, low shear and turbulent flow favour large platelet aggregate formation. The activated platelets generate thrombin and initiate coagulation. The flow then carries these fibrin-stabilized platelet aggregates into the gap resulting in occlusion of the gap and arrest of flow. B: A flat segment created along the inner wall of a conical plastic tube forms the basis of the technique, since it prevents the round steel ball bearing from occluding the lumen. When blood is added, it flows through the narrow gaps by the ball and exits in droplets into an adjacent collecting tube. The latter is trans-illuminated and a light sensor generates a signal whenever a drop of blood interrupts the light path. The instrument detects the time interval [d] between consecutive blood drops .