Prothrombin time (PT) and 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 7-10 seconds.
Prothrombin time is the time it takes plasma to clot after addition of tissue factor (obtained from animals). This measures the quality of the extrinsic pathway (as well as the common pathway) of coagulation.
The speed of the extrinsic pathway is very affected by levels of factor VII in the body. Factor VII has a short half-life and its synthesis requires vitamin K. Deficiencies in vitamin K, which can be caused by warfarin, liver damage, or an immature liver in newborns, result in an increased prothrombin time.
Prothrombin time can be measured roughly on whole blood (which is done in neonates), but is more commonly measured from blood plasma. Blood is taken into a Vacutainer containing liquid citrate. Citrate acts as an anticoagulant by using up all the calcium in a sample. The blood is mixed, then centrifuged to separate blood cells from plasma.
The plasma is put through a coagulation machine, which takes a sample of the plasma. An excess of calcium is added, which enables the blood to clot again. Tissue factor (or thromboplastin) is added, and the time the sample takes to clot is measured optically.
The prothrombin ratio is the prothrombin time for a patient, divided by the result for control plasma.
Sources: DiaPharma, Wikipedia