Lab Values, part 19: Activated Partial Thromboplastin Time (aPTT), Prothrombin Time (PT), & International Normalized Ratio (INR)

Hi, I'm Meris, and in this video, I'm going to be talking to you about three different anticoagulant lab values, activated partial thromboplastin time, aPTT, prothrombin time, PT, and international normalized ratio, INR. Let's go ahead and get started. So first, I want to talk to you just briefly about a big overview here. All three of these lab values are going to be ones that have to do with anticoagulation or coagulation. We are looking at different parts of the clotting cascade to see how much time it takes for blood to clot through the different components of the clotting cascade. The first one I'm going to talk to you about is aPTT, and it is usually referred to and thought of as separate. And then the last two that I'm going to talk to you about are PT and INR. And these two usually go together. They are often ordered simultaneously or on the same lab value. And we'll talk a little bit about why that is.

But first, I want to talk to you about aPTT, activated partial thromboplastin time. And this one right here is a measure in seconds how long it takes for the intrinsic and common pathways when we are trying to clot. So we are just looking at intrinsic and common pathways, not the extrinsic. And here we're going to measure in how many seconds it takes for the blood to clot through these pathways. The normal clotting time is 30 to 40 seconds. It shouldn't take too terribly long. 30 to 40 seconds is about what we would expect in an average person. However, many times this lab value is being run because somebody is on heparin therapy and we need to see if they are in a therapeutic range, meaning that the heparin is doing a good job and it's got their bleeding time exactly where we want it to keep their blood thin enough. Or is it not therapeutic? It's subtherapeutic, meaning we need a little bit more. Our blood is still clotting too much like normal. Or are we past the therapeutic level and now we're talking about a bleeding time in the many hundreds of seconds? Yikes. Now we have a big problem, right? So we have another expected value here, which is that when we are talking about the therapeutic range for heparin therapy, we're looking at 1.5 to 2.5 times the baseline. So one and a half to two and a half times what a patient's baseline is. And remember, their baseline should be between 30 to 40 seconds. So we would expect that this time would be about 50 to 100 seconds, okay? A little bit plus or minus, just depending on where that patient's personal baseline is and what lab we're looking at-- which facility is running the lab, I mean. But that is about where we would expect somebody to be for who is anticoagulated on heparin in a good range.

Now, what if I have an increased time? Either because I'm an average person who is not on anticoagulants, I'm not receiving heparin therapy, and my bleeding time is elevated, or because I am on heparin and my bleeding time is elevated. So of course, the causes include heparin therapy, right? If I give you heparin, it's going to elevate your time. DIC or disseminated intravascular coagulation. This is a very serious complication, a very serious coagulation issue. And we will see increased times with DIC. And liver disease. Again, this one stumps a lot of nursing students. Why liver disease? Liver is responsible for the production of different clotting factors. So if I have a faulty or diseased liver, then it's possible that I'm not going to be able to clot my blood appropriately, and I may see those increased times. This is just a key point, a reminder for you that if this is for a patient who's on heparin therapy and they have a markedly increased time and they need reversal, protamine sulfate is going to be the antidote for heparin. So this is the lab that I most commonly am monitoring when I have a patient on heparin therapy. And for me, in the emergency room setting, most commonly, this is someone who either is experiencing an NSTEMI and non-ST elevated myocardial infarction or somebody with a DVT or a pulmonary embolism. So that is what you need to know about aPTT.

Now let's move on to the rest. All right, so we've looked at the intrinsic and common pathways, but now we want to look at the extrinsic pathways. So now we're going to order a prothrombin time, a PT. And this is going to take a look at, again, in seconds, how long it takes for the blood to clot through the extrinsic and common pathways. The extrinsic and common pathways, we expect to see this blood clotting in about 11 to 12 and a half seconds. So much faster than the intrinsic pathway. And this is what we would expect for a patient at baseline, just walking around out in the community who is not anticoagulated. However, the reason that we often are drawing this lab is because we have patients who take warfarin. Now, I told you aPTT is what we check for a patient who takes heparin. A patient taking warfarin or Coumadin is more likely going to be the person walking around out in society. This is a PO medication that your patient can take at home. So this is very commonly a lab value that a patient on warfarin therapy is going to have monitored on a routine basis, really making sure that those levels are staying nice and therapeutic. So what is the therapeutic level for PT? It's going to be one and a half to two times the patient's baseline. So again, we are going to draw it before we start them on that anticoagulant therapy, and that will be their baseline. And then we expect their therapeutic range to be that one and a half to two times above that.

Now, what if we have elevated levels of our prothrombin time, our PT? Well, some possible causes include warfarin therapy because they're anticoagulated on warfarin. Again, DIC, disseminated intravascular coagulation, liver disease, vitamin K deficiency. Because again, remember we are talking about the extrinsic pathway. And if I don't have enough vitamin K, I'm not going to be able to clot my blood through that pathway, and we will see that increased time. And then just other clotting factor deficiencies. So if I'm struggling with some other-- maybe this is somebody with hemophilia, or maybe this is just some sort of other type of hematological clotting factor deficiency. Well, then they don't have the clotting factors. It's going to take much longer for blood to clot, and we're going to see that elevated bleeding time. Now, things to note here is that the vitamin K is the antidote for warfarin. So again, this is super important, and these things need to go together in your mind. A patient taking warfarin, we need to know that vitamin K is the antidote in case they were to be experiencing toxicity to their warfarin that needs to be reversed or possibly if they are experiencing some sort of a complication such as a brain bleed, we will need to reverse that as well.

All right. So now I'm going to talk to you about INR, which is slightly different, but these two things kind of go together. So INR stands for international normalized ratio. And what this means, it means that we're going to take our patient's PT result, the one I just talked to you about, their prothrombin time. We're going to take that result and we're going to compare it to a standardized patient. And that standardized patient is the international normal patient, okay? This is the one that we use to say, this is what a standardized patient should be. And there's a ratio that is done to compare our patient to the standardized control patient. And this creates a ratio. If I am exactly the same as that control patient, then my INR will be 1.0. It will be a one-to-one ratio, okay? Now, again, this is one of these things that we are going to be expecting to be close to 1.0, because I'm expecting you to be close to that standard patient if you are not anticoagulated. Now, the expected range for this lab value is going to be 0.8 to 1.1. There are no units associated with this, because again, remember, this is a ratio. This is not a time. This is not a measurement of an amount of something in your blood. It is a comparison between your patient and a standardized patient. So there are no lab values. I'm sorry. There are no units for this lab value.

The therapeutic range for warfarin is going to be 2 to 3. So that's a nice, easy one to remember. We're very thankful when that happens. So this is what I'm going to be looking at when I have a patient receiving warfarin and being anticoagulated. I want to make sure that we're not getting up high, high, high past that 3.0 mark. Again, again, again, please, I just want to really beat this into your heads. Protamine sulfate is the antidote for heparin. Vitamin K is the antidote for warfarin. Very important patient teaching that these patients who are on warfarin need to be maintaining a consistent intake of vitamin K, not meaning that we're not supplementing, we're not suddenly being carnivores and not having any leafy greens and our vitamin K level drops. We are maintaining a consistent intake so that we are not monkeying around with how that warfarin is working in the body.

All right, I'm so glad you saved until the end because I've got some quiz questions to test your knowledge of key facts provided in this video.

When monitoring a patient on heparin therapy, which lab value is most crucial to assess for therapeutic effectiveness and potential bleeding risk?

aPTT.

A patient on warfarin therapy is found to have an INR value of 3.5. What dietary recommendations might you consider to help stabilize their INR levels?

Encourage a consistent intake of vitamin K through the diet to decrease fluctuations.

If a patient's PT is prolonged without the use of anticoagulants, what potential underlying health conditions might be suspected?

Liver disease or vitamin K deficiency.

All right, that is it for this video. I do hope you found it useful. I'll see you in the next one. Thanks so much and happy studying.