Blood-tests Explained


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This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

INR


At a Glance

Why Get Tested?

A prothrombin time (PT) is a test used to help detect and diagnose a bleeding disorder or excessive clotting disorder; the international normalized ratio (INR) is calculated from a PT result and is used to monitor how well the blood-thinning medication (anticoagulant) warfarin (Coumadin®) is working to prevent blood clots.

When to Get Tested?

When you are taking warfarin or when you have unexplained or prolonged bleeding or inappropriate blood clotting

Sample Required?

A blood sample drawn from a vein in your arm; sometimes blood from a fingerstick

Test Preparation Needed?

None needed, although if you are receiving anticoagulant therapy, the specimen should be collected before taking your daily dose.

The Test Sample

What is being tested?

The prothrombin time (PT) is a test that helps evaluate a person's ability to appropriately form blood clots. The international normalized ratio or INR is a calculation based on 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 a person's sample of blood after substances (reagents) are added. The PT is often performed 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 PTT test evaluates those protein 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 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 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®).

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.

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.

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.

The Test