Prothrombin time gives an indication of the concentration of prothrombin in the blood. Figure 36-5 shows the relation of prothrombin concentration to pro-thrombin time. The method for determining pro-thrombin time is the following.
Blood removed from the patient is immediately oxalated so that none of the prothrombin can change into thrombin. Then, a large excess of calcium ion and tissue factor is quickly mixed with the oxalated blood. The excess calcium nullifies the effect of the oxalate, and the tissue factor activates the prothrombin-to-thrombin reaction by means of the extrinsic clotting pathway. The time required for coagulation to take place is known as the prothrombin time. The shortness of the time is determined mainly by prothrombin concentration. The normal prothrombin time is about 12 seconds. In each laboratory, a curve relating pro-thrombin concentration to prothrombin time, such as
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