The prothrombin time is a measure of extrinsic pathway activity. It assesses tissue factor, factor VII, and the final common pathway.
Causes of prolonged PT
- Warfarin (and other vitamin K agonists)
- Vitamin K deficiency
- Hepatosis
- DIC
Vitamin K deficiency and poor liver synthetic function results in reduced production of coagulation factors and has a more pronounced effect on the PT. The prothrombin time can be used a surrogate measure of liver function.
In theory, heparin should prolong the PT. However, most PT assays contain heparin binders to neutralise this effect. Additionally, all NOACs prolong the PT but the results are unreliable.