The prothrombin time (PT) and its
derived measures of prothrombin ratio (PR) and international
normalized ratio (INR) are measures of the extrinsic pathway
of coagulation. This test is also called "ProTime
INR" and "INR PT". They are used to determine the clotting
tendency of blood, in the measure of warfarin dosage, liver damage, and vitamin K status. PT measures factors I, II, V, VII, and X. It is used in conjunction with the activated
partial thromboplastin time
(aPTT) which measures the intrinsic pathway.
Normal range
The
reference range for prothrombin time is usually around 10-13 seconds; the
normal range for the INR is 0.8–1.2. Clinicians desiring therapeutic
anticoagulation may aim for a higher INR - in many cases ranging from 2-3 -
using anticoagulants such as warfarin.