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.