The
prothrombin time (
PT) — along with its derived measures of
prothrombin ratio (
PR) and
international normalized ratio (
INR) — are assays evaluating the
extrinsic pathway of
coagulation. This test is also called "ProTime INR" and "PT/INR". 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 (fibrinogen),
II (prothrombin),
V,
VII, and
X. It is used in conjunction with the
activated partial thromboplastin time (aPTT) which measures the
intrinsic pathway and common pathway.