The prothrombin time (PT) an its derived measures of prothrombin ratio (PR) and international normalized ratio (INR) are measures of the extrinsic pathway of coagulation. They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage and vitamin K status. The reference range for prothrombin time is usually around 12-15 seconds; the normal range for the 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.