Platelets and Coagulation Tests

Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Coagulation tests assess platelet count, prothrombin time (PT), and partial thromboplastin time (PTT) in the bloodstream. Explore a brief overview of how clotting works, learn about the three major clotting tests, learn how doctors interpret the test results, and discover hemophilia's impact on one famous family. Updated: 09/30/2021

Royal Hemophilia

Perhaps as a little kid you saw the animated movie Anastasia. It was a movie based on true events with a healthy dose of Hollywood spin. The real-life princess Anastasia had a brother named Alexei who suffered from a condition called hemophilia. This is a disease where the blood fails to properly clot, one where even a small scratch could turn deadly.

That's why in desperation, his mother, Empress Alexandra, turned to the notorious charlatan Rasputin in order to try and save Alexei's life. Thankfully, we don't have to rely on people like Rasputin anymore and can instead turn to science to diagnose clotting disorders via several tests mentioned in our lesson.

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  • 0:01 Royal Hemophilia
  • 0:48 Clotting: A Brief Overview
  • 3:16 PT, PTT, And Platelet Count
  • 4:47 Interpreting the Test Results
  • 8:50 Lesson Summary
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Clotting: A Brief Overview

OK, let's begin then. Before getting to the tests, it's necessary to appreciate at least the very basics of what clotting is and what it entails. Clotting is a composite of two general components. This includes platelets, which are quasi-cells that combine with clotting factors (which are proteins) to form a clot, a plug of sorts, in any area of bleeding.

If there was a deficiency in number or appropriate function of platelets or coagulation factors, then a person would be at an increased risk for bleeding. An area that is bleeding is like an open wine bottle turned upside down, pouring its red wine out. It's the platelets and coagulations factors that form the cork that stops the bottle from becoming empty.

Most of the clotting factors are made in the liver, and many of them are dependent on vitamin K in order to be synthesized. Specifically, factors II, VII, IX, and X are vitamin K dependent. All of these will be important points to remember for later.

The way clotting occurs is also dependent on two main systems, or pathways. They are termed the intrinsic pathway and the extrinsic pathway. It's simple to remember when each system is triggered. If some sort of damage occurs 'in'ternally, 'in' a blood vessel, then the 'in'trinsic pathway is activated. The intrinsic pathway involves clotting factors VIII, IX, XI, and XII. Otherwise, if something injures the blood vessel 'ex'ternally to the blood vessel itself, then the 'ex'trinsic pathway is initiated. The extrinsic pathway involves factor VII.

Hopefully, by looking at the first two letters in 'ex'ternal and 'in'ternal, you'll easily remember which system is triggered when.

These two pathways then combine to form one appropriately called common pathway that includes factors I, II, V, and X.

Of course, it's not difficult to understand that many times, these two pathways work together. If some kind of trauma to the body occurs, it's going to damage tissues and blood vessels. In that case, the intrinsic pathway clots blood in blood vessels and the extrinsic pathway clots any blood that may have escaped into the tissues.

And with that lowdown on the coagulation cascade out of the way, it's time we look at interpreting blood tests that rely on this information.

PT, PTT, and Platelet Count

Prothrombin time, also known as PT, is a measurement that assesses the extrinsic and common pathways of the coagulation cascade. You may be familiar with PT if you are on warfarin (aka Coumadin) therapy or know someone who is. PT is used to measure the effectiveness of this therapy.

Partial thromboplastin time, abbreviated as PTT, is a measurement that assesses the intrinsic and common pathways of the coagulation cascade. You might know this test if you are on heparin therapy as it's used to monitor the effectiveness of this treatment. PTT, for all practical purposes and this lesson, is also known as activated partial thromboplastin time, abbreviated as aPTT.

To try and remember which test measures what factors, think about the fact that the intrinsic pathway has more factors involved in it and PTT has more letters than PT. It's a very simple trick, but it works!

Another important measurement with respect to coagulation that we need to look for is our platelet count. If blood tests reveal there are abnormally low levels of platelets in the blood, we term this thrombocytopenia. A thrombocyte is a technical term for a platelet. A 'thromb'ocyte helps to form a thrombus, which is a blood clot. Hopefully that little tidbit helps you remember the interrelationship between these words.

Interpreting the Test Results

Along with thrombocytopenia, another red flag that tells us something is wrong in our body with respect to clot formation is if PT or PTT is prolonged on a blood test, meaning it's taking longer than normal for the blood to clot.

Again, I want to repeat something important. If there is a decrease in the number or function of platelets in our body or if there is a decrease in the number of one or more coagulation factors, as evidenced by a prolonged PT or PTT, then that spells trouble! Keep this in mind as we move along here.

Now that we've got that settled, it's time we interpret some blood work. Let's say our blood test comes back with the following information: a normal PT, PTT, and thrombocytopenia. This means that something is causing our platelet numbers to drop, increasing our risk of bleeding out. This could be due to:

  • Increased destruction of platelets, as a result of things like drugs or immune thrombocytopenic purpura, a condition where your own body destroys its own platelets.
  • Decreased production of platelets. Examples of why this may occur include nutritional deficiencies, genetic disease, or something like bone marrow destruction as a result of cancer.

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