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Prothrombin Time and INR

The prothrombin time (PT or protime) is the actual time, measured in seconds, for an anticoagulated sample of plasma or whole blood to clot after it is added to a thromboplastin reagent. Thromboplastin is a mixture of tissue factor and calcium chloride. The PT assesses the coagulation factors of the extrinsic pathway (factors VII, X and V) and the factors in the common pathway (prothrombinase [a complex of activated factor X, activated factor V, and calcium which acts as the substrate for prothrombin], prothrombin, thrombin and fibrinogen). Physicians may request a PT to evaluate a patient:

  • Investigate a history of a bleeding tendency prior to invasive surgical procedures
  • Prior to intravascular cardiac surgery or other procedures that interfere with coagulation
  • Determine the etiology of abnormal bleeding
  • Assess liver function.
  • Monitor warfarin therapy

The prothrombin time may be prolonged by deficiency of a singl

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