How I Teach the Coagulation Cascade

Associate professor in the Department of Medicine, Division of Hematology and Oncology, at the University of North Carolina School of Medicine in Chapel Hill

In this issue, Alice Ma, MD, shares her advice for teaching the coagulation cascade – to hematologists and non-hematologists, medical students, and attendings.

Why is the coagulation cascade such a pain to teach – and to learn? The first and most obvious problem: Coagulation is written in Roman numerals. Right off the bat, we Arabic numeral–users are at a disadvantage. Next, the factors were numbered in order of their discovery, and not in order of their function; and, as blood researchers learned more about coagulation, they realized that there is not, in fact, a factor II, or IV, or VI. We thought there was, but there’s not.

And, then there is the more frustrating problem: The model of the coagulation cascade helps explain abnormalities in blood clotting test results, but it does not reflect the patient’s biology. If it did, then hemophiliacs shouldn’t bleed, factor Xa inhibitors such as low-molecular-weight heparin (LMWH) should prolong clotting times, and patients with factor XII deficiency should bleed horribly. In reality, none of these things is true.

As one could expect, this infuriates learners.

My job, as a medical educator, is to help them out. So, how do I make it a bit easier? I use simple learning tricks like mnemonics that make the coagulation cascade easier to remember. My goal is to establish a base level of knowledge so that learners can figure out simple problems; if a more complex problem arises, they can come and find us.

They just need a little push to start them off on the right foot, and they can find their way through the rest of it.

Enter the Coagulation Made Easy presentation (click to download) – complete with mnemonics, visual aids, and simple diagrams. I’ve borrowed (okay, stolen) bits of this from other smart people like Michael Lapetina, and I’ve lent it to many others.

How I Like to Teach

Coagulation is complicated and, many times, the people who are teaching coagulation are enthusiastic about the topic – which is good. What’s not so good is when they try to tell the trainees everything they know. In the realm of coagulation, that’s a lot.

Personally, I like teaching with mnemonics and analogies. Nothing is off-limits or too juvenile if it can help a student learn. I care less about the delivery of the material than I do about whether the learner grasps the material.

My teaching philosophy is: “Don’t make topics any harder than they need to be.” I like to make things simple, and bring them to the level of the trainee. For example, I take advantage of the Roman numerals by teaching learners that “X” marks the spot where two pathways converge, and that the point at the bottom of a “V” conveniently fits into the cleft at the top of an “X”!

Who knew?

Also, the numbers of the Common Pathway happen to correspond to all of the U.S. bills under $20 – if we continue to use the $2 bill, which I hear is making a comeback, anyway.

Some people might turn their nose up at learning devices like mnemonics but, for a difficult and complicated subject, I say bring it on. Give me something I can jam into my head, and we’ll work on the specifics later. Does that make me intellectually lazy? I hope not, but if it means I can get medical students, fellows, and even orthopedic surgeons to know the coagulation cascade, then I will gladly be called lazy.

Yes, we want our learners to understand the nuances of a subject, but how many people are going to drill down for 12 hours to understand coagulation at that level of detail? I’m betting no one – and certainly no time-pressed medical students.

The method I developed is practical, hands-on, and – probably its greatest strength – not scientific at all.

And, the truth is, this approach works. If my obituary ends up in my local paper, it will be because I have gotten literally tens of thousands of people to understand the coagulation cascade, some of them for minutes at a time. I use this technique for learners at all stages (medical students, residents, fellows, attendings of all ranks) and all disciplines (anesthesia, surgery, OB-GYN, etc.).

Coagulation Cascade Conundrums

Why do I prefer this “non-scientific” approach? Because of the aforementioned stumbling block: The coagulation cascade doesn’t make sense in real-life biology.

Most learners are quick to comprehend my mnemonic because it helps explain the coagulation cascade – and it soothes their frustration for a bit because it makes sense and they actually understand it. Then, unfortunately, I have to burst their bubbles and say, “Okay, great, you get it. But this doesn’t reflect real life.”

For example, if there are two equivalent methods for generating factor Xa from factor X – meaning just as much thrombin is produced in factor Xa from factor VII – then why is factor VIII needed on the other side of the cascade? If the cascade reflected real-life biology, then hemophiliacs shouldn’t bleed. And why doesn’t factor XII matter? It leads to a prolonged partial thromboplastin time (PTT), but it doesn’t have any effect on biology.

Another sticking point: When heparin is used, it will block thrombin, inhibit factor Xa, and prolong the PTT and, to a lesser effect – the prothrombin time (PT). In a PT assay, heparinase is used, so heparin is degraded, and the effect on PT is understandable. However, this doesn’t explain why LMWH does not affect the PT and PTT times. LMWH inhibits thrombin-generating factor X; without factor X, thrombin is unable to turn fibrinogen into fibrin. Why doesn’t that affect the PT and PTT?

Hematology can be hard.

As hard as we might try to find an answer, there are certain facts that escape our understanding. We have to tell medical students something they never want to hear: “Even though we don’t understand this, you still have to know it.”

This part is difficult for medical students, but acknowledging that roadblock, pointing at it and saying, “Yes, this part sucks,” is good. The important distinction to make is that no one understands it. Trust me, I have asked plenty of people, and no one can explain this to me.

How do we break this to students? Admit that it’s a mystery to all of us. Think about it this way: Can I explain the innerworkings of my car’s internal combustion engine? Do I know what a carburetor does? Not really, but I still drive my car. For all intents and purposes, it’s a miracle.

Why I Teach the Coagulation Cascade

Knowing the principles of the coagulation cascade helps explain test results, but it doesn’t explain what we see in patients with bleeding disorders. It is a frustrating concept – one that requires patience both to learn and to teach.

This is especially important when we, as hematologists, are answering questions from clinicians in other disciplines, such as anesthesiologists and cardiac surgeons. These specialists often send for PT and PTT tests, but they often don’t know how to interpret the results. So, if they gain a basic understanding about the coagulation cascade and can understand the results without calling for a hematologic consult, I’m satisfied.

For others who want a better and deeper understanding of coagulation, there’s the elegant cell-based model of hemostasis outlined by Hoffman and Monroe.1 It is a beautiful description that helps refine the theory and understanding of coagulation. I use it to explain coagulation to trainees who want more insight into biology.

The approach I developed may not provide a comprehensive understanding of the complexities of the coagulation pathways, but it helps people remember where the various coagulation factors fit into the cascade and identify abnormal test results associated with factor deficiencies. It’s slightly more than a passing familiarity and, as I like to tell people, knowing the coagulation cascade is guaranteed to help you on at least two board exam questions.


Hoffman M, Monroe DM 3rd. A cell-based model of hemostasis. Thromb Haemost. 2001;85:958-65.