INTERNATIONAL NORMALIZED RATIO (INR)

Prothrombin time (PT) and its derived measures Prothrombin Ratio (PR) and International Normalized Ratio (INR) are the measures of extrinsic pathway of coagulation. They are used to monitor the warfarin dosage, liver damage and vitamin K status. The reference range for PT is usually 12- 15 seconds and for INR is 0.8-1.2. PT measures factors II, V, VII, X and fibrinogen. It is used in conjunction with the activated partial thromboplastin time (APTT) which measures the intrinsic pathway. Prothrombin time is the time taken by the plasma to clot after addition of tissue factor (obtained from animals). It measures the quality of the extrinsic pathway (as well as the common pathway) of coagulation.