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.

Coagulation tests in 400 words or less

Making a blood clot involves three steps:

  1. blood vessel constriction
  2. platelet plug formation, and
  3. fibrin formation (also called coagulation).

There are lab tests that evaluate steps 2 and 3 (nobody talks much about poor step 1). Let’s look at the main tests that are used to evaluate step 3.

If you think back to the basics of the coagulation cascade, you might recall that there are two arms – an extrinsic arm and an intrinsic arm – which come together in the final common pathway, which ends up turning fibrinogen into fibrin. When somebody is bleeding, and you think it’s due to a coagulation problem (as opposed to a platelet problem), it’s helpful to know what part of the cascade is screwed up. That helps you figure out what’s wrong with the patient (is it hemophilia? or liver disease? or coumadin overdose?).

There are two main tests for evaluating the cascade: one for the extrinsic arm (the INR) and one for the intrinsic arm (the PTT). There are other tests too – but those will have to be for another post.

1. The INR

In the olden days, this test was called the prothrombin time (or PT), and it was extremely variable from lab to lab. Now, the lab applies a mathematical correction to the PT to make the results more consistent. The new name for the nice standardized PT is the INR, or “international normalized ratio.”

Whatever. What you do in this test is add thromboplastin (which acts like tissue factor, the thing that kicks off the coagulation cascade in vivo) to patient plasma, and wait to see how long it takes for fibrin to form. This test (for reasons we’ll have to discuss in another post, since we’ve limited this one to 400 words) measures the extrinsic pathway, which is that arm of the coagulation cascade that involves tissue factor, factor VII, and the final common pathway (X, V, II, and fibrinogen).

2. The PTT

The PTT, or partial thromboplastin time, is performed by adding just some phospholipid to the patient’s plasma and waiting to see how long it takes to form fibrin. It’s called the “partial thromboplastin” time because initially, it was found that by adding a part of thromboplastin to a test tube, you could activate fibrin formation. It turns out that the part of thromboplastin people were adding was just phospholipid, and that thromboplastin consists of both phospholipid and tissue factor. This test measures the intrinsic pathway, which is that arm of the cascade involving factors XI, IX, VIII and the final common pathway.

Whew. Okay, that was 431 words. Close enough.

What does the bleeding time really measure?

white-bleeding-heart

Here’s a very good question about the diagnostic use of the bleeding time.

Q. I’m currently studying heme for boards and came across a practice questions that used platelet count, bleeding time, PT and PTT values to differentiate between certain diseases/problems. I was just wondering how in both Vitamin K deficiency and liver disease you can get an increase in PT and PTT but the bleeding time doesn’t change…I guess I figured that bleeding time would have to increase.  Can you explain this to me?

A. Yeah, that does sound weird, you’d think the bleeding time would change – but actually, the bleeding time is only a measure of platelet function. It really has nothing to do with coagulation!

I kind of think of it like this: the platelet plug is the first thing to form, and that is enough to stop the bleeding from the incision made at the beginning of the test. The coagulation cascade happens next, and the status of that won’t be apparent in the bleeding time results. The patient might have some more bleeding later if their coagulation system is really screwed up…but the bleeding time assay will be done by then. In reality, it probably happens a little more concurrently than that (platelet plug is followed very closely by fibrin formation – the two probably even overlap a bit), but I think it’s a good way to remember the concept.

The same reasoning fits with the way that people with coagulation factor disorders bleed (as opposed to patients with platelet disorders). People with platelet abnormalities tend to bleed spontaneously into mucous membranes without much provocation (probably because they’re having a hard time forming that initial platelet plug) whereas patients with coagulation factor abnormalities, like hemophilia, tend to have deep, severe bleeds that happen after some time has elapsed (because they form the initial platelet plug okay, but they can’t seal it up with fibrin very well, so they end up bleeding later on).