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Prothrombin Time with INR

Also known as

PT, Pro Time, INR

Testing Basics


Why do you need to get tested?

This test is ordered to detect and diagnose an excessive clotting disorder or a bleeding disorder. INR refers to the International Normalised Ratio, which is calculated from a PT result. INR is used to gauge if the blood-thinning medication warfarin is working well enough to prevent the development of blood clots.

When do you need to get tested?

The prothrombin time is ordered when you are on warfarin medication, or if you have prolonged and unexplained bleeding, or inappropriate blood clotting.

Which kind of sample is required?

This is a blood test. The blood test requires a sample drawn from a vein in your arm. In some cases, blood is drawn from a finger stick.

Do you need to prepare for the test?

There is no preparation needed in general. If you are on anticoagulant therapy, the blood sample must be collected before you take your daily dose.

What is being tested Prothrombin Time (PT) with INR?

This prothrombin time/PT test evaluates the ability of your body to form blood clots appropriately and adequately. The INR is in turn based on the results of the prothrombin test, which is used to monitor people that are being treated by warfarin, a blood-thinning medication. The prothrombin tests measure the total seconds it takes for a clot to form in the blood after reagents, which is a substance used in performing a laboratory test, are added. The prothrombin test is usually supplemented with a partial thromboplastin time. The tests are used in conjunction to measure and evaluate the amount and function of coagulation factors which are proteins that are an essential part of proper blood clot formation. When an individual is injured and bleeds out, hemostasis, the clotting process begins. Hemostasis consists of a chain of chemical reactions, which is called the coagulation cascade, in which clotting or coagulation factors are triggered one after the other, eventually forming a clot. In order to do this effectively, there must be a sufficient amount of each coagulation factor. In addition, they must function normally. This allows normal clotting  to take place. Too few coagulation factors may lead to excessive bleeding, and heavy amounts can lead to an excess of clotting.

  • Breaking down the test

There are two pathways that can lead to clotting in a test tube during the lab test. These are known as extrinsic and intrinsic pathways, which merge into one common pathway in order to carry out clotting. The PT test is used to evaluate if the coagulation factors in the extrinsic and common pathways of the coagulation cascade work together seamlessly. These include: factors I, also known as fibrinogen, II is also known as prothrombin, and V, VII, and X. This test may also be done with the PTT, which examines the clotting factors that are part of common and intrinsic pathways. These include XII, XI, IX, VIII, X, V, II, and I. In addition, they also evaluate prekallikrein and high molecular weight kininogens.

How is the test used ?

The PT is measured in seconds and is compared to the normal range, which mirrors PT values in people who are healthy. Since normal ranges fluctuate due to the variety of reagents used in lab tests, INR is used to balance it out. INR is calculated on the basis of the PT test result for people that are taking warfarin. The INR calculation allows the doctors to adjust changes in PT reagents and allows them to compare a variety of results from various laboratories. Laboratories tend to report both the prothrombin test values and INR values when the PT test is performed. The INR, however, is only applicable to people that are on blood-thinning medication. Both values are used to monitor how effective warfarin is. Warfarin is used to thin out the blood and prevent inappropriate clotting. Usually, your doctor prescribes the tests to measure how well warfarin thins the blood. After the results are received, the dose is adjusted accordingly in order to make sure that it effectively prevents clotting without excessive bleeding. The PT is used in conjunction with PTT to look into excessive bleeding or clotting disorders. Using the results, the doctor can identify what bleeding or clotting disorder may be present. These tests are not diagnostic by themselves but usually provide information on whether or what further testing may be needed. These tests may be ordered alongside platelet counts, coagulation factor tests, fibrinogen tests, von Willebrand factor tests, and Lupus anticoagulant testing.

When is the test ordered?

The test may be ordered by your practitioner when a person is taking warfarin in order to ensure that the drug is performing optimally. However, it may also be ordered when the person has signs or symptoms of excessive bleeding like easy bruising, unexplained bleeding, bleeding gums, chronic conditions like liver disease, a blood clot in the vein or artery, or DIC. PT may also be ordered before surgery when the surgery carries a serious risk of blood loss or when the person has a clinical history of arbitrary bleeding.

What does the test result mean?

In general, laboratories report PT results that have been adjusted to the INR for people on warfarin. These people should have an INR of 2.0 to 3.0 for basic "blood-thinning" needs. For some who have a high risk of a blood clot, the INR needs to be higher - about 2.5 to 3.5. Results for people that are not on warfarin are evaluated in accordance with normal ranges set by the lab. A PT, which is prolonged, indicates that the blood clotting process is slow. This is indicative of diseases like liver disease, vitamin K deficiency, or coagulation factor deficiency. Usually, the PT result is interpreted with the PTT result in order to gauge what the condition is.

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