International Normalized Ratio

  • Ratio of the patient's prothrombin time to a normal (control) sample, raised to the power of ISI value for the analytical system used.
    • The prothrombin time is the time it takes plasma to clot after addition of tissue factor (obtained from animals such as rabbits, or recombinant tissue factor, or from brains of autopsy patients). This measures the quality of the extrinsic pathway (as well as the common pathway) of coagulation. The speed of the extrinsic pathway is greatly affected by levels of functional factor VII in the body. Factor VII has a short half-life and the carboxylation of its glutamine residues requires vitamin K. The prothrombin time can be prolonged as a result of deficiencies in vitamin K, warfarin therapy, malabsorption, or lack of intestinal colonization by bacteria (such as in newborns). In addition, poor factor VII synthesis (due to liver disease) or increased consumption (in disseminated intravascular coagulation) may prolong the PT.
  • Provides a standard way to measure likelihood of bleeding during surgery.
  • Normal range for a healthy person is 0.9 - 1.3
  • Patients on warfarin therapy 2.0 - 3.0
  • A low INR such as 0.5 indicates a high chance of bleeding.
  • Routine implant surgery generally safe between 2 - 4