When the blood clots and is spun, the gel rises to separate the serum from the cells. The smaller tubes collect about 5 ml, and the large tubes about twice that. Your body makes 30 ml of new blood each day. Some tubes contain an anticoagulant so the blood does not clot. Other tests use serum, so the blood is allowed to clot and then separated.
Some tests require special preservatives in the tube.
A. Usually a stick to obtain blood hurts only while the needle is breaking the skin, not much and only for a second. It helps to look away. A CBC is a complete blood count . We also look at the types of white cells that you have. Sometimes we can detect mono this way.