Also known as: Activated Partial Thromboplastin Time (aPTT) and Prothrombin Time (PT), PT and aPTT, PT and PTT.A
The PT test is usually measured in seconds and compared to the values in healthy individuals. Because the reagents used to perform the PT test vary from laboratory to laboratory and even within the same laboratory over time, the normal values also will fluctuate. In order to standardize results across the U.S. and the world, a World Health Organization (WHO) committee developed and recommended the use of the Internationalized Normalized Ratio (INR) with the PT test for patients who are receiving the blood-thinning medication warfarin (Coumadin). The INR is a calculation that adjusts for changes in the PT reagents and allows for results from different laboratories to then be compared. Both the PT and INR values are reported on this test.
The PTT may also ordered at intervals to monitor unfractionated (standard) heparin anticoagulant therapy. The prothrombin time (PT) test measures how long it takes for a clot to form in a blood sample. In the body, the clotting process involves a series of sequential chemical reactions called the coagulation cascade, in which “clotting” factors are activated one after another and result in the formation of a clot. Prothrombin is one of the coagulation factors and produced by the liver. The PTT may be ordered, along with other tests such as a PT, when a patient presents with unexplained bruising or bleeding, a thromboembolism, an acute condition such as disseminated intravascular coagulation (DIC) that may cause both bleeding and clotting as factors are used up at a rapid rate, or with a chronic condition such as liver disease.
