For demonstration purposes, I sometimes need to prepare DNA from whole blood through a very "rough" procedure. The blood is collected with a butterfly needle in 10 ml-heparin tubes, and then I transfer this 10 ml of blood into a 50 ml-falcon tube. To this, I add 40 ml of red cell lysis buffer, I incubate on ice for 10 minutes, and then I centrifuge on 3000 rpm for 10 minutes. Afterwards, you should see a nice white pellet at the bottom of the falcon tube. This is where it goes wrong too often to be a freak accident. In about 50% of the cases, I don't get a nice pellet, but a thick red clot is formed in the bottom of the falcon tube, with red liquid above it. It is like the heparin didn't work and the blood still clotted. But how can that be, because there was heparin in the tubes, and from the same batch of collected blood, there will be some falcon tubes that do have a nice pellet. It's like it's a completely random event, but very annoying when you need to demonstate something. Can anybody shed some light on this mystery??