Prothrombin time

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Also known as: PT; International normalised ratio; INR
Formal name: Prothrombin time
Related tests: Activated clotting time (ACT), activated partial thromboplastin time (aPTT or PTT), prothrombin consumption time (PCT), fibrinogen, coagulation factors

At a Glance

Why Get Tested?

To check how well clot-preventing medications (anti-coagulants) are working to prevent blood clots, to investiate the clotting tendency of blood, liver damage and vitamin K status

When to Get Tested?

If you are taking an warfarin-related anti-coagulant drug or think you may have a bleeding disorder

Sample Required?

A blood sample drawn from a vein in the arm. Your doctor will advise you on how frequent this testing will be.

Test Preparation Needed?

None

The Test Sample

What is being tested?

The test measures how long it takes for your blood to begin to form clots. Prothrombin is a plasma protein produced by the liver. Clotting is caused by a series of chemical reactions (coagulation cascade), including the conversion of prothrombin to thrombin. The test used to measure this clotting factor is called prothrombin time, INR or PT.

How is the sample collected for testing?

Blood is collected by needle from a vein in the arm.

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

No test preparation is needed.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.