When you have a condition that requires adjustment of your blood’s coagulation levels, you always want to know that your level is in the right range. Anything otherwise could be dangerous. But for all those times you’ve had your blood tested, have you ever paused to wonder what you’re actually testing? Do you know what the results really mean and how they’re determined?
Naturally, your doctor explained the essentials of anticoagulation therapy and INR testing to you, but the more you know, the better you can take control of your health and have the peace of mind that your INR is in a safe range.
Understanding What INR Measures
A common misconception is that your INR – or Internal Normalized Ratio – measures the amount of warfarin in your blood. Actually, it’s not the amount of warfarin that you’re measuring but the effects of warfarin on your blood.
Warfarin is an anticoagulant, which means it decreases your blood’s ability to coagulate, or form clots. INR monitoring helps you and your doctor determine whether your blood’s clotting ability is in a safe range. If your INR is higher than your prescribed range, that means your blood coagulates too slow, which could lead to bruising and bleeding. If it is lower, your blood coagulates too fast, which may result in the formation of a dangerous blood clot.
That’s why it’s important to always know your INR numbers. Testing your INR regularly at your prescribed frequency will enable you to identify as soon as possible when your INR is out of range. Your doctor will discuss with you recommended adjustments to your medications, diet, or other factors to get your INR back to a safe range.
How Your PT and INR Are Calculated
You may have heard the terms “INR” and “PT” test used in similar ways. PT stands for prothrombin time. Prothrombin is a component of your blood plasma that assists with your blood’s ability to clot. A PT test measures in seconds how long it takes your blood to clot. For individuals who are not taking an anticoagulant drug like warfarin, a normal PT time is 11 – 13.5 seconds, according to the U.S National Library of Medicine at MedlinePlus.gov. For individuals who do take warfarin, the clotting process will likely take longer.
Because there can be some variability between PT tests depending on the lab and the exact testing process they use, the International Normalized Ratio (INR) is a way of standardizing your numbers so results from any lab can be compared accurately. The ratio is calculated using the following formula:
INR = (patient PT/mean normal PT)ISI
Your PT test result (patient PT) measured in seconds is divided by the average PT of a normal blood sample (mean normal PT). Then the international sensitivity index (ISI) is applied to the result to determine your INR. ISI is another way of taking testing variables into account so your INR can be accurately compared to other results.
How Your Doctor Determines What the Right INR Range Is for You
Your prescribed target INR range generally depends on the condition your anticoagulation medication treats. Two of the most common medical conditions treated by warfarin are atrial fibrillation and venous thromboembolism, both of which typically require a target INR range of 2.0-3.0. The target INR range may be higher for mechanical heart valve patients. The target INR range may vary between 2.0 and 3.5 depending on a number of factors. If you are generally healthy and are not being treated with warfarin therapy, a typical INR is around 1.0. Always discuss with your doctor what INR range is right for you.
Always Know Your Numbers with Home INR Monitoring
Alere Home Monitoring makes it easy for you to test your INR regularly. The home testing routine allows you to test yourself at home—or even when you’re away from home—and report your results to your monitoring team. Your doctor will contact you if a dosing adjustment is needed. With home INR monitoring, you are in control of your numbers, and a medical professional always has your back. Enjoy the peace of mind and healthy living that comes with knowing your numbers and what they mean.