Blood samples for testing can be taken either from a vein (which carries blood to the heart) or from an artery (which takes blood away from the heart).
If only a few drops of blood are needed (for monitoring blood sugar in diabetes, for example) it is enough to make a small prick in the tip of the finger and then squeeze the blood out.
Most blood tests are taken from a vein, commonly from those around the elbow. First a cord (tourniquet) is tied around the upper arm to make the vein prominent. It may be a bit tight, but this makes it much easier to take the test.
The site of the injection is cleaned with spirit and then a needle is put into the vein. The needle will be attached either to a low pressure blood test bottle, or to a syringe where the plunger is pulled back to create low pressure. When the necessary amount of blood has been extracted, the needle is removed and a little ball of cotton wool is held over the wound. This should be pressed for one to two minutes before applying a sticking plaster.
If blood is taken from an artery, it is usually extracted from the wrist where there is an artery that is very close to the skin. This may be slightly uncomfortable, as the artery wall has more pain nerves in it than the vein wall.
After taking blood from an artery it may be necessary to hold a ball of cotton wool on the place where the injection was made for about five minutes to stop any bleeding.
Some people are very sensitive to needles and the sight of their own blood and may feel faint when a blood sample is taken. This is not uncommon and can be reduced by sitting or lying down while the sample is taken. If you feel faint or think that you might feel faint, immediately tell the person who is taking blood.