Why do the INR and PTT measure different pathways?

test tubeCoagulation questions seem to come up all the time! Here’s a good one from one of our readers.

Q. In both the PT and PTT we add thromboplastin, right? So how come the PT measures the extrinsic pathway and the PTT measures the intrinsic pathway?

A. This is a great question because it really gets at the underlying concepts of the PT (INR) and PTT. When I was a medical student, I never really thought about why the INR only measured the extrinsic pathway and the PTT measured only the intrinsic pathway. I just memorized the substance added to the test tube in each test, and the pathway the test measured. Later on, though, I realized I didn’t have a clue as to why the tests measured the pathways they did.

Before we get into the reasoning behind the tests, a quick correction is in order. We don’t add thromboplastin in both the INR and PTT. In the INR, you add thromboplastin, and in the PTT you add phospholipids (not thromboplastin). It turns out thromboplastin is a substance that contains both phospholipids AND a tissue-factor-like substance. That’s why they call the assay the “partial thromboplastin time” – because you only need to add part of the thromboplastin reagent (the phospholipid part) to get this test to run.

To understand why the PT measures just the extrinsic pathway and the PTT measures just the intrinsic pathway, you need to know what activates these pathways in the body. The extrinsic pathway is activated by tissue factor. The intrinsic pathway can be activated by a bunch of things, the most important of which is thrombin.

Why the INR measures the extrinsic pathway

To get blood in a test tube to form fibrin along the extrinsic pathway, you need to add some tissue-factor-like substance. Also, since you removed the platelets and calcium before running the test, you need to add those things back into the test tube (the coagulation system needs a phospholipid surface, normally provided by platelets, and calcium to run). Thromboplastin is a substance that contains both phospholipids and a tissue-factor-like substance. Add thromboplastin and some calcium, and the blood in the test tube will form fibrin via the extrinsic pathway.

Why the PTT measures intrinsic pathway

To get blood in a test tube to form fibrin along the intrinsic pathway, you don’t need to add any tissue-factor-like substance (if you do, the extrinsic pathway will be activated!). All you need to do is add back what you took out of the blood (phospholipids and calcium), as well as something like silica or kaolin to activate the intrinsic pathway (normally, thrombin does this job in vivo), and you’ll form fibrin along the intrinsic pathway. This is actually why the intrinsic pathway was named the way it was: everything you need to get the pathway to run is “intrinsic” to the blood. The extrinsic pathway requires something “extrinsic” to the blood (tissue factor) for it to run.

Bottom line

The INR activates the extrinsic pathway because in this test you add thromboplastin (which contains both a tissue-factor-like substance and phospholipids) to the test tube. The PTT activates the intrinsic pathway because in this test you add just phospholipids to the test tube – and without tissue factor around, fibrin is formed along the intrinsic pathway.

How come the extrinsic and intrinsic pathways are named that way?

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Q. How did the “extrinsic” and “intrinsic” coagulation pathways get their names? It seems counter-intuitive.

A. Excellent question!! And one that a lot of students have asked in class. “Intrinsic” sounds like the important pathway that happens in the body, and “extrinsic” sounds like one that might happen in the lab, or outside the body. But that’s not at all what the names mean!

The two pathways were named that way because of the way blood clots in a test tube in the lab – not because of the way the pathways act in the body. Both pathways are totally necessary for coagulation to proceed in the body – let’s get that straight right off. But in the lab, you can do each pathway separately (the INR, or the PT, measures the extrinsic pathway, and the PTT measures the intrinsic pathway).

Extrinsic pathway

If you want to get the extrinsic pathway to run in a test tube, you have to add something extrinsic to the blood. Remember: the extrinsic pathway is kicked off by tissue factor combining with VIIa. Tissue factor is not normally present in the blood (it’s in little closed-up particles, or it’s in the subendothelium, or it’s in inflammatory cells…it’s a mysterious little substance). So if you want to get blood to clot in a test tube via the extrinsic pathway, you have to add tissue factor (which is extrinsic to the blood) to the test tube.

Intrinsic pathway

If you want to get the intrinsic pathway to run, you don’t have to add anything – everything that it needs is already in the blood. Remember: the intrinsic pathway is kicked off in the body by thrombin (and, less importantly, by other stuff, like bradykinin and high molecular weight kininogen). So everything you need for that pathway is already in the test tube; all the factors are intrinsic to the blood.

Of course, for both pathways, you have to replace the calcium and phospholipid surfaces you took out of the blood, because the coagulation factors need calcium and a phospholipid surface to work. For coagulation tests, you draw blood into a blue-topped test tube which contains a chelating substance that takes out the calcium in the blood (otherwise the blood would clot before you even got back to the lab). You also remove the platelets before running the tests (platelets provide a phospholipid surface for the coagulation factors to sit on in the body). But calcium and a phospholipid surface are normally present in the blood – so by adding them to your test tube, you’re not really adding anything new – you’re just replacing what you took out.

Ah, coagulation. Always a challenge! If you want to read a nice summary of coagulation (if I do say so myself), check out Clot or Bleed: A Painless Guide for People Who Hate Coag. It goes through both the intrinsic and extrinsic pathways, describes how they actually fit together in the body (which, strangely, is something nobody seems to talk about), and gives you a creative (if weird) way to remember which is which.