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 addÂ 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.
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.
Q. What is the difference between FDPs and D-dimers? (more…)
Making a blood clot involves three steps:
- blood vessel constriction
- platelet plug formation, and
- 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.
Here’s a very good question about the diagnostic use of the bleeding time. (more…)