First, a quick review of how a blood clot is formed

Here’s how you make a clot: 

1. Constrict the affected blood vessel
2. Form a platelet plug
3. Form fibrin (using the coagulation cascade) to seal up the platelet plug 

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?).

The two main coagulation lab tests

There are two main tests for evaluating the cascade: one for the extrinsic arm (the PT/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 PT/INR

The prothrombin time (PT) is performed by adding thromboplastin to the patient’s plasma, and seeing how long it takes to make fibrin (as soon as you see the first strands of fibrin form, the test is over, and you measure the result in seconds.

Thromboplastin contains a tissue-factor-like substance, and therefore it just measures the extrinsic pathway. The annoying thing about thromboplastin is that its strength varies considerably from manufacturer to manufacturer. So the PT done at one hospital (using manufacturer A’s thromboplastin) will be significantly different than the PT done at another hospital (using manufacturer B’s thromboplastin). Dumb.

To deal with this annoying issue, someone figured out how to make some mathematical calculations that take into account each manufacturer’s thromboplastin strength. Now, you just do your PT assay, add on those mathematical calculations, and you wind up with something called the International Normalized Ratio (INR). This means that you can do the PT with anybody’s thromboplastin, and you’ll wind up with a result (the INR) that removes that variability. Super important. 

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. You have to add in a little “contact factor” like kaolin to activate XI to XIa – but other than that, that’s it!

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.