Transcription
It's Medicosis Perfection. L is resuming our discussion about bleeding and coagulation disorders. In the previous video, we talked about the fibrin degradation products. Today, I'll talk about the D-dimer, a subset of the fibrin degradation products. It happens when the length-stabilized fibrin is being degraded by plasmin. With that being said, let's get started. [Music]
Hemostasis has many steps. We are still stuck in number four: plasminogen to plasmin. Thank you, TPA. Plasmin will degrade fibrin. Fibrin degradation products and the stabilized fibrin into D-dimer. And here's the whole story in one slide, which I have discussed in a previous video.
In the last video, we talked about fibrin or fibrinogen degradation products, or fibrin or fibrinogen split products. They come from fibrin or fibrinogen, respectively, thanks to plasmin. They are water-soluble; the kidney can get rid of them. In the ICU, you have increased fibrinolysis; you'll have increased fibrin degradation products and D-dimer. And in diseases such as primary hyperfibrinolysis, you have increased fibrinolysis, and we will have increased fibrin degradation products and D-dimer. By the way, D-dimer is a subset of fibrin degradation products, which is even cooler. Is the fact that I have 50 hematology cases waiting for you guys on patreon.com/medicalsystem. They are difficult.
So here is fibrin before factor XIII. Factor XIII will cross-link the fibrin by adding covalent bonds. Here is the fibrin without stabilization, and here's the fibrin being stabilized. How did factor XIII do it? It added some cross-links between the fibrin fibers. So how do you stabilize any strands? You'll link them by putting connections between them to make them even stronger. This is what factor XIII is all about. For example, by the way, how do you make collagen fibers stronger? You link them to increase their tensile strength. We call this process covalent bonding, cross-linking, or cross-bridging.
When you see D-dimer or elevated D-dimer in the plasma of a patient, it's an evidence that this patient has linked fibrin fibers floating around in the blood, and it's an evidence that he or she had stabilized fibrin clot in their system. Fibrin, by plasmin integration, plants the stabilized fibrin. My plasmin, the D-dimer. By the way, "di" means two, because here are two strands, or two fibrin fibers, connected together by a cross-bridge. So D-dimer assay is a lab test; it detects only fibrin fibers that have a link. These fibrin fibers are not going to be detected by the D-dimer because they don't have a link. The fibrinogen fibers are not going to be detected because they are fibrinogen; they are not fibrin, let alone linked. So if these are the fibrin degradation products, D-dimer is included within them; it is a subset of the fibrin degradation products.
So let's talk about D-dimer. It's a fibrin degradation product. Yep, it is. Okay, so sorry. It results from degradation of stabilized, covalently cross-linked fibrin. This was a mouthful. When you see D-dimer, it's an evidence that there was a stabilized fibrin here. D-dimer assay detects only the fibrin fibers that have a link.
Now let's play a game. You bring up a piece of paper; try to answer these three questions. Then I'll pause, let you answer them, then we'll come back. Okay, pause. Now let's do the answers. First question: Does D-dimer assay detect fibrinogen degradation products? In other words, if you have fibrinogen degradation products in a patient, will you have an elevated D-dimer? And the answer here, of course, is nope; you will not, because D-dimer is only present as a result of degradation of the linked fibrin, not the crazy fibrinogen. Cool.
Next: Would a patient who has broken down a platelet-rich thrombus in his or her coronary artery have increased D-dimer? And the answer is—or are you ready? He said, "Are you ready?" I have a technical problem here, sorry—and the answer is yes. Well, you thought this is a platelet thrombus that doesn't contain fibrin. Hey, when you hear the word thrombus, it means we had some platelets, and these platelets are linked together by fibrin fibers, and then they will get stabilized by factor XIII, and then, after being stabilized by factor XIII, now you have an issue; you have a coronary thrombus, which can kill you. Will you expect increased D-dimer? The answer is yes.
Third question: Would a patient who has DIC show increased D-dimer? And the answer here is going to be—drumroll, please—the answer here is yes.
So clinical uses of the D-dimer assay: First, to rule out DVT and PE, excellent, because these are clots. If they are clots, we expect fibrin fibers that have been linked by factor XIII. Cool. In selective populations, which means when the pretest probability is low. Let me give you an example: We have a 21-year-old young, healthy male. He goes to the gym three times every week; he pays his taxes; he's perfectly healthy; no family history of any clots; no history of clots whatsoever; he's living the dream. And the D-dimer rules out DVT in this kind of patient. The answer is yes. Why? Because the pretest probability is low. The probability of DVT in this patient before running the D-dimer test is low. So when the D-dimer test comes back negative, it has essentially ruled out the DVT in this patient because D-dimer is very sensitive—95% of cases. It's not specific, but very sensitive.
Let me give you another example of a high pretest probability when D-dimer's not going to work. Let's have a 49-year-old female, morbidly obese, has been pregnant seven times before; she is on birth control pills; she had a major surgery three weeks ago, and or like five days ago to be exact, and she had a history of multiple major surgeries. She's morbidly obese; she doesn't leave the bed. Now her ankle is swollen, and it's tender, and it hurts, with a positive sign for DVT. You run the D-dimer test; it came back negative. Does the D-dimer test rule out DVT for this patient? The answer, of course, is no. Who cares about the negative D-dimer? All of that scenario, all of the physical, the history, everything in this case points to DVT. I'm not going to believe D-dimer in this case; I'm going to believe my eyes and the history. The pretest probability is high, so D-dimer is not going to work. That's what it means.
The most sensitive test to diagnose DIC is D-dimer. This was the question of last video, so it has diagnostic value, then a risk stratification value. It stratifies the risk of recurrent venous thromboembolism, especially in women. So you will say she is less risky, she is more risky, low risk, high risk, etc. After giving a patient TPA for any of these diseases—coronary artery thrombosis, ischemic stroke—remember, only ischemic, not hemorrhagic—mesenteric venous thrombosis, and renal artery stenosis, we follow up using the D-dimer. If the D-dimer's high, there was a clot.
Now I'll leave you with these five questions. These are kind of difficult, and the answer to these questions are available on patreon.com/medicosis. Thank you for watching. Subscribe and hit the bell to get notifications. Follow me on Facebook and Instagram. Get my notes and get my 50 hematology cases by going to patreon.com/medicosis, and I'll send you my bloody Dropbox links because we're talking about hematology here. Thank you for watching. Until next time, be safe, stay—have the end—study hard. This is Medicosis Perfection, as far as medicine makes perfect sense.