If you have been taking warfarin or Coumadin, you know that you need to regularly have your blood tested. The test you have done is called the INR (sometimes referred to as the “Protime” or “PT/INR”). INR stands for “International Normalized Ratio” and PT stands for “Prothrombin Time.” Simply stated, the INR is a reflection of how long it is taking your blood to clot.
Originally, only the PT was used to measure prolonged clotting due to warfarin. This was challenging for both patients and physicians because the results could vary depending on where the test was run and results from different laboratories couldn’t reliably be compared. So, in the mid-1980s, the World Health Organization endorsed a mathematical equation that standardized this type of testing. Now, using the INR, blood tests for warfarin patients could be compared between laboratories, states and even countries.
PTnormal is the amount of time in seconds that it takes normal blood (no warfarin) to clot—generally 10-12 seconds.
PTtest is the amount of time in seconds that it takes your blood to clot.
ISI is the “International Sensitivity Index” and is assigned to the reagents being used to run the test (This is generally 1.0).
Your home monitor (or a laboratory) does the math for you and the only important number for you, as a patient, is the INR!
An INR of 1.0 is a “normal” INR—the clotting process has not been significantly slowed down by warfarin. Most people who don’t take warfarin have an INR of 1.0. However, there are some medicines (such as the newer oral anticoagulants) and disease states (like liver disease) that can prolong the INR without warfarin.
Since the INR is a reflection of time, the higher the number gets, the longer it is taking the blood to clot. For most people who take warfarin, the target INR range is 2.0-3.0. There are people who need higher target ranges of 2.5-3.5 and sometimes 3.0-4.0. Ultimately, your target range is determined by the reason you take warfarin, your other medical history, and your physician.