Why does blood not clot when it’s in a surgical drain?

We put this to Professor Adrian Newland:

Adrian - It may eventually clot but at the operation site blood gets activated so the clotting factor's actually get stimulated and removed so the blood is able to clot and it will build up around the wound and cause damage and prevent healing. It's sucked out. Chris - When you say clotting factors, Adrian, what actually are they?

Adrian - They're little cells called platelets but also chemicals within the blood called clotting factors.

Chris - How do they work?

Adrian - They work by becoming activated by raw tissue and they form a little spider's web that brings the blood together. Chris - And it grabs blood cells, presumably? It forms this meshwork and you don't bleed any more. If you've got oozing through that just plasma and things what Thomas might have been seeing in his drain was just plasma minus these things that have already been removed?

Adrian - Yes. It might have been exudate that oozed out and was being sucked away.