I think Ironman meant that one single RBC isn't enough protein to test for something, like to run a Western or whatever (though Westerns wouldn't give you anything relating to DNA), not that you need pure protein. There are other components in the blood which can be used for DNA verification though. That's what I interpreted from Ironman's post too, in the same way you need a certain amount of blood to be able to get a blood test, you may also need a certain amount of source material to run a DNA or any other sort of test, a single cell probably doesn't provide enough matter to do anything, I guess...