Abstract: The apparatus includes a single-use compartment assembly which includes a reservoir for blood and a needle element by which blood is obtained from the user. A vacuum system is actuated which draws blood through the needle element into the blood reservoir. An ultrasound signal is directed through the reservoir. There is no return ultrasound signal until there is sufficient blood in the reservoir. When the blood reaches a given level, it contacts a clotting agent present in the reservoir to begin the clotting process. The ultrasound signals are transmitted and return until clotting has been completed, as determined by a pre-established increase in the two-way propagation time relative to unclotted blood. The prothrombin time is then calculated and compared with an acceptable range.