What is Prothrombin Time-PT?
Prothrombin time (PT) is a blood test that measures the time it takes for the liquid portion (plasma) of your blood to clot.
A related blood test is partial thromboplastin time (PTT)
Prothrombin is a protein produced by your liver. It is one of many factors in your blood that help it to clot appropriately.
The prothrombin time test is one of several tests used to screen people waiting for liver transplants. That screening — known as the model for end-stage liver disease (MELD) is a scoring system for assessing the severity of the chronic liver disease.
The prothrombin time (PT) is a test that helps evaluate your ability to appropriately form blood clots. The international normalized ratio or INR is a calculation based on the results of a PT that is used to monitor individuals who are being treated with the blood-thinning medication (anticoagulant) warfarin (Coumadin®).
A PT measures the number of seconds it takes for a clot to form in your sample of blood after substances (reagents) are added. The PT is often performed along with a partial thromboplastin time (PTT) and together they assess the amount and function of proteins called coagulation factors that are an important part of proper blood clot formation.
In the body, when there is an injury and bleeding occurs, the clotting process called hemostasis begins. This process involves in part a series of sequential chemical reactions called the coagulation cascade, in which coagulation or “clotting” factors are activated one after another and result in the formation of a clot. There must be a sufficient quantity of each coagulation factor, and each must function properly, in order for normal clotting to occur. Too little can lead to excessive bleeding; too much may lead to excessive clotting.
In a test tube during a laboratory test, there are two “pathways” that can initiate clotting, the so-called extrinsic and intrinsic pathways. Both of these then merge into a common pathway to complete the clotting process.
- The PT test evaluates how well all of the coagulation factors in the extrinsic and common pathways of the coagulation cascade work together. Included are: factors I (Fibrinogen), II (Prothrombin), V, VII, and X.
- The PT/INR may be done at the same time as a PTT, which evaluates the clotting factors that are part of the intrinsic and common pathways: XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).
The PT and PTT evaluate the overall ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient in quantity or not functioning properly, the test results will be prolonged.
The PT is usually measured in seconds and is compared to a normal range that reflects PT values in healthy individuals. Because the reagents used to perform the PT test vary from one laboratory to another and even within the same laboratory over time, the normal ranges also will fluctuate. To standardize results across different laboratories in the U.S. and the world, a World Health Organization (WHO) committee developed and recommended the use of the Internationalized Normalized Ratio (INR), calculated based on the PT test result, for people who are receiving the anticoagulant warfarin (Coumadin®). Warfarin is prescribed for people with a variety of conditions, such as deep vein thrombosis (DVT) and some cardiovascular diseases (CVD) like atrial fibrillation to “thin” their blood and prevent inappropriate clotting.
The INR is a calculation that adjusts for changes in the PT reagents and allows for results from different laboratories to be compared. Most laboratories report both PT and INR values whenever a PT test is performed. The INR should be only applicable, however, for those taking the blood-thinning medication warfarin.