A prothrombin time test measures how quickly your blood clots. Sometimes called a PT or pro time test, a prothrombin time test uses a sample of your blood.

Prothrombin is a protein produced by your liver. It is one of many factors in your blood that help it to clot appropriately.

If you take a blood-thinning medication such as warfarin (Coumadin), your prothrombin time test results will be expressed as a ratio called the international normalized ratio (INR).