DefinitionPartial thromboplastin time (PTT) is a blood test that looks at how long it takes for blood to clot. It can help tell if you have bleeding or clotting problems.Alternative NamesAPTT; PTT; Activated partial thromboplastin timeHow the test is performedThe health care provider uses a needle to take blood from one of your veins. The blood collects into an air-tight 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 many seconds it takes for the blood to clot.How to prepare for the testThe health care provider may tell you to stop taking certain drugs before the test. Drugs that can affect the results of a PTT test