There are several kinds of anesthesia.
General anesthesia is like sleeping.
You don't feel any pain or remember anything. The blood to be tested is drawn out of a vein with a syringe or taken by pricking a finger. A finger-stick sample is more likely to be contaminated by contact with the surface of the finger than is a sample taken by putting a sterile needle into a vein. Many different tests can be done on the blood, and a few of the most common ones are described here. More complex tests can be done to find out what chemicals are present in the blood.