How To Teach Coagulation Tricks Of The Trade

by Donna Castellone, MS, MT (ASCP) SH • May 09, 2022



The interpretations below are provided by Donna Castellone, MS, MT (ASCP) SH for Aniara Diagnostica.


I have been teaching coagulation for well over thirty years. I have taught everyone from hematology fellows, medical students, medical technology students, high school students and even a third-grade class. Needless to say, you need to be mindful of your audience. I have always been very clear that my focus is laboratory-based medicine, I teach what happens in the laboratory in coagulation, what you see from your results and what it may or may not mean for the patient.

Coagulation is complex, many people do not like it, they choose to not understand it, so I have tried over the years to make it as simple as possible so that people can think about it logically (well sort of) and help them remember what happens where and when in the cascade. Having an understanding of what happens when enables you to know how to proceed with coagulation testing. So, nothing is off limits, using mnemonics or algorithms as tools is very helpful.

 

Where to start?

The factors are in Roman numerals which is very intimidating to people. There are not a lot of characters to memorize: just I = one, V= five, X= ten – I do not recommend mixing using both Roman and Arabic numerals- it can be very confusing- because factor II in Roman looks like 11 in Arabic. Let's looks at a very simple laboratory-based cascade, this is not what happens physiologically but what we use to determine laboratory testing.

Coagulation Cascade

This is the simple cascade I like to use to teach. Here are the hints that I use:

For the factors of the Common pathway remember: Run to the 5 &10 (that would be I, II, V and X) since it is common, it is measured by BOTH the PT and aPTT

The Factor for the Extrinsic pathway is ONLY Factor VII. And is measured by ONLY the PT.

The remaining factors and found in the Intrinsic pathway and that would be VIII, IX, XI and XII. These are measured by the aPTT.

We know that factors are produced in the liver, except for factor VIII which is partially made in endothelial cells. As a result, in liver disease you would expect to see that all factors would be decreased, except for factor VIII which is elevated. Factor VIII is an acute phase reactant which is elevated in times of inflammation, stress, pregnancy and infection. Other acute phase reactants in coagulation are fibrinogen and von Willebrand factor.

How does this information help you? Now you understand what to look for when a patient has a prolonged PT and or aPTT and a possible factor deficiency- it's that simple. But what if the patient is on warfarin? Let's look, the Vitamin K deficiency factors are: II, VII, IX and X, Protein, C and Protein S. Warfarin works by rendering the Vitamin K factors non-functional based on their half-lives. Which factor has the shortest half-life? Well Factor VII (7UP and OUT) is the shortest 4-6 hours and Factor II (The LAST 2 GO) so based on the simple cascade which test will be the first to prolong on a patient on warfarin? Factor VII is measured in the extrinsic pathway, so the PT will be prolonged.

Based on all of the information above, when looking to see if a patient has liver disease, many doctors will check factor V (since it is not a Vitamin K dependent factor) and it will be decreased in liver disease and factor VIII expecting it to be elevated in liver disease since it is not produced there and is an acute phase reactant. To determine if patients are in a therapeutic range for warfarin, clinicians will look at a Factor II level, because a factor VII level will fall quickly, Factor II will fall as a patient approaches therapeutic levels.

What about heparin as an anticoagulant? The most sensitive test to determine the presence of heparin is a thrombin time. If you want to detect if heparin is on board, do a thrombin time it will be prolonged at low levels of unfractionated heparin. Unfractionated heparin can be monitored by the aPTT as long as the laboratory confirms the therapeutic range for its reagent lot. Low molecular heparin at higher levels will prolong your aPTT but it cannot be monitored using the aPTT. If it needs to be monitored it should be done using an anti-Xa level. It is very important that these samples are collected, processed and tested within an hour because if the plasma is allowed to sit on the cells, platelet factor 4 will neutralize heparin and falsely decrease heparin levels.

Now that you have the basics, you can understand about a mixing study. When you have a sample that has a prolonged screening test (PT and or aPTT) you mix that patient sample with a sample of pooled normal plasma. That pooled normal plasma should contain normal levels of all of the factors. It is important to know the level of the factors in the pooled normal plasma (NPP), remember when you mix it with patient plasma you are actually diluting the activity of the factor in half- so if the level was at 100% in the original NPP it is now at 50%, so if you begin with a level that is at 60% when you perform the mix, it may not correct your patient sample. Let's start with a prolonged aPTT of 67 seconds, you perform a 1:1 mix and it corrects into your laboratories criteria for acceptance for correction to 32 seconds (normal reference range 28-35 seconds) NPP= 33 seconds. This correction would point you in the direction of a patient having a factor VIII, IX or XI deficiency. What about a FXII deficiency? That is also possible, however, those patients do not bleed, they are more likely to thrombose. Many clinicians will specifically request a FXII as a separate test, while others laboratories may include it as part of a prolonged aPTT work up.

If a mixing study doesn't correct, you would then look for an inhibitor. There are specific inhibitors to factors in which can cause a patient to bleed like in hemophilia, or non-specific inhibitors which will be seen in patients that present with a lupus anticoagulant (LA). Patients with a LA generally do not bleed, they are more likely to thrombose. In the laboratory, they present as an antibody to phospholipid. Our reagents are made of phospholipids. This is so confusing to so many people, try to stick with me! Many laboratories choose to use a reagent that is insensitive to a lupus anticoagulant. This aPTT reagent will have a high concentration of phospholipids which will overwhelm the presence of an LA, so their aPTT test will not pick up an LA, the aPTT will not be prolonged. Other laboratories will choose to use a lupus sensitive reagent which has a lower concentration of phospholipids which will allow the antibody to manifest and prolong the aPTT.

 

Conclusions:

These are the basic understanding that clinicians and laboratorians need to have in coagulation. This is just the tip of the iceberg, but it is a good start to being able to determine where the problem is and how to proceed with testing.

 


REFERENCES TO HELP YOU LEARN:

How I Teach the Coagulation Cascade Alice Ma, MD
How I Teach the Coagulation Cascade | ASH Clinical News | American Society of Hematology (ashpublications.org) April 2016

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

Shrestha SK. Simple Coagulation Cascade with Mnemonics [Internet]. Epomedicine; 2017 Jan 17 [cited 2022 Apr 5]. Available from: https://epomedicine.com/medical-students/simple-coagulation-cascade-mnemonics/

Chaudhry R, Usama SM, Babiker HM. Physiology, Coagulation Pathways. [Updated 2021 Sep 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482253