fractionationQ. Quick question for you about serum vs. plasma: I wrote in my notes during class that serum is plasma without the clotting factors. But then in another lecture, the professor talked about coagulation tests, and he said you add reagents to plasma and then wait for fibrin formation. Did I get it backwards in my notes? Is plasma just serum without clotting factors?

A. No – you were right the first time, in your notes: plasma has clotting factors, serum does not. In other words, serum is just plasma minus the clotting factors.

In the tests we talked about, we add reagents to plasma (which has all the coagulation factors in it already) to kick off the coagulation cascade and see how long it takes to make fibrin. Actually, a more accurate way to say it is that all the coagulation factors are present except tissue factor. That comes from endothelial cells and other places; it’s not just free-floating in the blood (otherwise you’d constantly be running the cascade and making fibrin!).

You might be wondering why, if all the coagulation factors are already there, the plasma doesn’t just clot on its own. The reason for that is because when we draw blood for coag tests, we draw it into a tube that has citrate in it (to remove calcium, which is necessary for the cascade to run). We also take out platelets once we get back to the lab (removing the phospholipid surface that the cascade also needs).

When we do the tests, we add back in the phospholipid and calcium – and if you don’t add anything else, then the coagulation cascade will start happening (it proceeds along the intrinsic pathway, in that case). If you also add a little tissue factor (in the form of thromboplastin), then the extrinsic pathway will be activated. That’s actually the basis for the development of the PT/INR vs. the PTT: add nothing (except phospholipid and calcium), and fibrin is formed along the intrinsic pathway. Add some thromboplastin (along with the phospholipid and calcium), and that kicks off the extrinsic pathway.