For Patients

Blood Tests for Blood Thinners

Taking a blood thinner can be risky. If the effect is too strong, you risk bleeding. But if the effect is too weak, you risk clotting. So knowing that the dose you are taking is correct is important. For this reason, we use blood tests for blood thinners. But we don’t always use these tests. In fact, there are blood thinners that a patient just takes in the correct dose, and we trust the effect is what it needs to be.

Blood Thinner Types

The purpose of this article is not to discuss blood thinner types. But in order to explain blood tests for blood thinners, you need to know something about the medications themselves.

Oral Blood Thinners

For the longest time the most common blood thinner people took was Coumadin. Another name for Coumadin is Warfarin. Basically, Coumadin works in the liver. It inhibits clotting factors there and that thins the blood.

But since the mid-2000’s other blood thinners have become common. These include medications such as Xarelto (rivaroxaban), Eliquis (apixaban), Pradaxa (dabigatran) and Savaysa (edoxaban). These medications also inhibit clotting factors. But different than Coumadin, they are more specific. Each medication inhibits just one specific factor.

Injectable Blood Thinners

There are multiple types of injectable blood thinners. I will not write about all of them. But I should mention two.

The first, is heparin. Heparin is a short-acting anticoagulant. This means the effects starts quickly but we can also reverse it rather quickly. It is given through the vein continuously and usually we administer it in the hospital.

The second blood thinner is low-molecular weight heparin. True, it is a type of heparin. But we inject it under the skin in one, two or three injections during the day.

Blood Tests for Blood Thinners

There are two purposes for blood tests for blood thinners. The first, is to check if the blood thinner is even in your body. This can be important sometimes. For instance, before surgery. Obviously, usually we would like to avoid surgery on blood thinners. But the second, is to check the magnitude of the effect of the blood thinner. Sometimes, that is routine (like with Coumadin). But sometimes, we need to know if we are using the correct dose. For instance when we prescribe blood thinners in obese people.

We can test for the presence of blood thinning effect with simple lab tests that most labs will have. For example, we can use tests such as thrombin time (to check for the effect of dabigatran) and prothrombin time (PT) for the effect of Coumadin.

Testing the Strength of Blood Thinning

Testing the strength of blood thinning is different. This needs to be accurate. You may know that testing for Coumadin has been around for a very long time. The test is called the international normalized ratio or INR. A higher INR means the blood is “thinner”. When we prescribe Coumadin, we indicate the INR range we want the patient to be in. A typical range will be 2-3. So, if the INR is too low, we know that the Coumadin effect is too weak. But if the INR is too high, we understand that the effect is too strong.

The INR test is especially important because Coumadin interacts with many medications, with food and with the bacteria in our gut. People who take Coumadin need routine testing to make sure the medication is working the way it is supposed.

However, patients who take Xarelto, Eliquis, Savaysa or Pradaxa do not routinely test for the blood thinner effect. These medications have fewer interactions compared to Warfarin. When they were first tested, the companies that make them learned the effect of different doses. Then, they decided on a certain dose that works for most people and that is the dose we prescribe to patients.

Now, if we really need to, we can test the effect of these medications. For instance, if we are trying to figure out why someone who took them still developed a blood clot. But, these tests are not available in most places. They are typically developed locally, in more advanced medical centers. Interpreting the results can be tricky.

So, how do you know they are working? When you take a blood pressure medicine, you can test your blood pressure. Or, if you take a cholesterol pill, you test your lipid levels. But here, you just need to trust the effect and follow with your doctor for any signs of new clots or bleeding.

Dr. Ido Weinberg

Dr. Ido Weinberg is a Vascular Medicine specialist at Massachusetts General Hospital. He is President-Elect of the Society for Vascular Medicine. Dr. Weinberg treats hundreds of patients with blood clots every year. He publishes extensively on blood clots and he speaks frequently about blood clots in international conferences.

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