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PT and INR

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Also known as: Prothrombin Time; Pro Time; Protime
Formal name: Prothrombin Time and International Normalized Ratio

The Test Sample

What is being tested?

The prothrombin time (PT) test measures how long it takes for a clot to form in a sample of blood. In the body, the clotting process involves 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, there are two "pathways" that can initiate clotting, the so-called extrinsic and intrinsic pathways. Both of these then merge into a common pathway (like the shape of a "Y") to complete the clotting process. In one of the final steps of the clotting cascade, prothrombin (also called Factor II) is converted into thrombin, but this factor and step is not the sole focus of the PT test.

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 test evaluates the overall ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient or dysfunctional, the PT will be prolonged.

The PT test 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 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 people who are receiving the anticoagulant warfarin (COUMADIN®).

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 are now reporting both PT and INR values whenever a PT test is performed. The INR is only applicable, however, for those taking the blood-thinning medication warfarin.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or, sometimes, from a fingerstick.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. If a person is receiving anticoagulant therapy, the specimen should be collected before the daily dose is taken.