Understanding the PT-INR Test

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Understanding the PT-INR Test

Marie B. Walker

January, 2004



If you have ever undergone oral anticoagulation therapy, you have most likely heard of the PT-INR test, but you may not understand what it is and/or what it means. This article will attempt to explain the PT-INR test.

First let's talk about the PT test. PT stands for prothrombin time. It is a measure of how quickly blood clots. The traditional method for performing a PT test is to have your blood drawn and sent to a lab. At the lab, a substance called a reagent is added to your blood. The reagent causes the blood to begin clotting. The PT result is the time in seconds that is required for the blood to clot.

There are a variety of reagents that can be used when a PT test is performed. Since each of these reagents works a bit differently, a PT result obtained with one reagent cannot be compared to a PT result obtained with another reagent. To account for the different reagents, the result of a PT test must be converted into standard units that can be compared regardless of the reagent used. These standard units are known as INR units.

INR stands for International Normalized Ratio. As its name suggests, one INR result can be compared to another INR result regardless of how or where the result was obtained. So, the INR is really just the standard unit used to report the result of a PT test.

There are a few things worth mentioning about the INR. First, an individual whose blood clots normally and who is not on anticoagulation should have an INR of approximately 1. The higher your INR is, the longer it takes your blood to clot. In other words, as the INR increases above a given level, the risk of bleeding and bleeding-related events increases. On the other hand, as the INR decreases below a given level, the risk of clotting events increases.


The ideal target INR range will vary from person to person depending on a variety of factors such as the reason for taking anticoagulants, other medical conditions, and a number of other issues. Your anticoagulation clinician will determine the appropriate INR range for you. The most common INR target range for someone on warfarin is somewhere between 2.0 and 4.0. INRs of 5 or more typically are avoided because the risk of bleeding increases significantly at INRs above 5.


Once your clinician has established an appropriate target INR range for you, he/she will adjust your anticoagulation therapy to keep your INR in the target range. If your INR rises above the high-end of your range, your risk of bleeding and bleeding related events may start to increase. If your INR falls below the low-end of your target range, your risk of forming a blood clot may increase.


Numerous factors can affect your INR, so it is important that you keep your clinician up-to-date on any changes in your life. For example, you should notify your clinician before starting new prescription or non-prescription drugs, including herbal remedies. Also, you should notify your clinician of changes in your diet, exercise regimen, alcohol use, tobacco use, etc. Good communication with your clinician will help you keep your anticoagulation therapy on track. For more information about oral anticoagulation therapy, please review the About Warfarin section of our website.