DefinitionProthrombin time (PT) is a blood test that measures the time it takes for the liquid portion (plasma) of your blood to clot.See also: Partial thromboplastin time (PTT)Alternative NamesPT; Pro-timeHow the test is performedThe health care provider uses a needle to take blood from one of your veins. The blood collects into an airtight container. You may be given a bandage to stop any bleeding. If you are taking a medicine called heparin, you will be watched for signs of bleeding.The laboratory specialist will add chemicals to the blood sample and see how long it takes for the plasma to clot.How to prepare for the testYour doctor may tell you to stop taking certain drugs before the test. For example, blood thinners can affect the results of this test.Do not stop taking any medicine w