PT INR stands for Prothrombin Time and International Normalized Ratio. The INR is a ratio that was created to set international standards for measuring prothrombin time accurately.
The formula is as follows, with ISI standing for the International Sensitivity Index:

Prothrombin time is the time it takes blood to clot after adding a tissue factor. Since the tissue factor varies between manufacturers, the ISI was implemented to compare the tissue factor used for the test to the international reference tissue factor. The result is a measurement of the extrinsic pathway of coagulation.

Prothrombin time can be measured to find the cause for abnormal bleeding or bruising, check for low levels of Vitamin K (necessary to make prothrombin), and to check levels of other blood clotting factors, since the lack of clotting factors can lead to bleeding disorders such as hempohilia. Another main reason for PT INR testing is to evaluate the efficiency of warfarin, an anticoagulant used in the prevention of thrombosis, as well as to check for liver damage.

Each year an estimated 800 million PT INR tests are performed.