Lab Values, part 20: D-Dimer

Hi, I'm Meris, and in this video, I'm going to be talking to you about the lab value D-dimer. Let's go ahead and get started. So today we're going to talk about the lab value D-dimer, which is actually one of my favorite ones to teach about. I just think it's really fascinating and I think it's a really interesting way of approaching this lab. So what is D-dimer? D-dimer is a protein fragment that gets broken off when a blood clot is broken down by the body. And now keep in mind, this is happening all the time. Whether you realize it or not, somewhere in your body, you have a clot. You have these constant little micro injuries. You're constantly tearing up your blood vessels. You're getting bumps and scrapes and bruises and all of these things. So I'm not talking necessarily about a huge clot. I'm just talking about a clot, okay? So this happens all the time. All the time we are making and breaking down clots. So at any given point in time, we would expect that there's going to be some amount of D-dimer in my blood, right, just kind of at baseline. But I don't want there to be too much. So we have to decide kind of what our normal is.

And so our cutoff here is that we say a D-dimer should be less than 0.4 mcg/ml. So again, this gives us a little wiggle room so that we can say, yes, it is normal to have some bumps and scrapes and cuts and clots that are being broken down, but not so much that we're concerned, right? So that cutoff point is 0.4. We want to be less than that. And we do have a cool chicken hint here to remind you, which is that I like to dim-- like D-dimer. I like to dim the lights very low, almost to zero, almost to zero, right? So it just helps you remember it should be very low. We want it to be as close to zero as possible. Now, we talk about these as being positive or negative when we run a D-dimer. And I just want to give you some information about how we use it in the hospital setting in particular, especially in the emergency room. So if you come into the emergency room and you're telling me, "Oh my God, I have this pain in my left calf and your calf is red and it's swollen and it's painful." and it walks like a DVT, it talks like a DVT, right? We're looking at this and I'm thinking, "Oh, this is a DVT." This is a lab that might be ordered to see if that's right. And if that lab comes back and it is positive, meaning it is greater than 0.4 mcg/ml, we are going to need to investigate that further, okay?

So when we are talking about a D-dimer, there's multiple things that can be going on here. I, for instance, D-dimer is a useless lab value when it comes to me because I have an inflammatory disorder that causes me to have connective tissue problems, which means that I am constantly covered in bruises all the time, very innocuous bruises, but bruises are going to make clots too that will need to be reabsorbed and broken down by the body. So for me, it is always positive. And this is something that I know just because of my personal medical history. But if I walked in off the street and said something is not right, and they ran a D-dimer and it came back elevated, they would say, "We need to investigate this further because an elevated D-dimer indicates that there may be a blood clot in the body." The thing to know here is it is not diagnostic. It is not saying, "Yes, this patient has a pulmonary embolism." It is not saying that I have a DVT. It's not telling you anything, any information about the cause of that elevation or the possible location of that clot, it is just saying, "I'm elevated." And if I'm elevated, further workup is needed. And that can be in a couple of different ways. It could be through an ultrasound, such as a venous duplex where they're going to scan the veins in my legs to see if I have a DVT. It could be done through a CT angiogram of the chest, sometimes called a spiral CT, where they will be looking with dye at the blood vessels of my chest to see if there is any kind of a clot. This could also be done with a VQ scan. Maybe I'm allergic to the dye that would be needed for the CT angiogram, so they can do something called a VQ scan, where they are comparing ventilation with perfusion, and they are able to see, is there an area where it is not perfusing very well, there could be a clot there.

So for me, as an ER nurse, it is important for me to know that D-dimer is not one of these labs that is a part of my hospital policy that I can order by myself. And there's a reason for that. And if you think back to what I just told you, the reason is that if it comes back positive, well, now the provider is obligated to do more follow-up on it. And again, there's lots of reasons that it could be elevated that are very innocuous. Maybe that person has a bruise somewhere, or maybe they have an autoimmune condition. But now this person needs maybe a CT scan, and a CT scan exposes them to ionizing radiation, which I really want to try to limit. So for my hospital policy, this is not a lab that can be included in an order set for the nurse. This is one that must be ordered by the provider because it is on them to have to follow up with a CT scan. So they are the ones to make that decision of, "Am I concerned enough about this to order this test?" So just a little clinical tidbit for you, and your hospital policy may differ, so refer to your policy, but that's how mine works.

Now, here's the thing about a negative result. And this is so interesting because I can't think of many other labs that are like this. If your D-dimer comes back negative-- let's say that same patient, red calf, swollen calf, it's painful to the touch, all of these sorts of things. Let's say their D-dimer comes back negative. Well, we can actually say that this is going to rule out a blood clot. It doesn't just make me feel better that maybe there's not. It's actually used to say, if the D-dimer is negative, then we are, I believe it's like 99.8% confident that there is no significant deep vein thrombosis, pulmonary embolism, et cetera. Because think about it. If I'm as close to normal as I can be, do I have a clot that's being broken down somewhere like in my leg or in my chest? Probably not, right? The bigger the clot, the higher that D-dimer is going to be. So this is one of those few labs that when it comes back negative, we're able to say, take that off the list. It's not a blood clot. And that is really cool. And that's great information to be able to give to your patient.

All right, I'm so glad you stayed until the end because I'm going to test your knowledge of key facts providing this video with a quiz question.

A negative D-dimer level is most commonly used to rule out which conditions due to its sensitivity?

Thromboembolic events such as deep vein thrombosis or pulmonary embolism.

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