Blood tests are done for different reasons. They can check your general health, or look for particular substances in your blood.
How are they done?
The blood is usually taken from a vein, generally in your arm at the elbow. A vein is one of the blood vessels that carry the blood to the heart.
To take the blood easily, you’ll need to have a tight band put around your arm called a tourniquet. A small needle is then inserted into your vein. Attached to the needle is either a small blood bottle about the size of an adult's finger (that draws the blood off under a low pressure), or a syringe (that has a plunger that’s pulled back to take the blood). At least 20 minutes before the blood test, some cream can be put on the area. This acts like a local anaesthetic and helps stop the blood test hurting. If you don’t have any cream, the needle feels a bit like a sting when it goes in. Once the sample has been taken, the needle is removed and the tourniquet is taken off.
A lot of people don’t like to see needles, or worry about seeing blood being taken and can feel faint. You’ll be sitting down when you have the blood test, but if you do feel faint just tell the person who’s taking your blood.
A piece of cotton wool is put over the skin where the blood was taken and you’ll be asked to press on it for a short time (about one minute). Then a plaster will be put on.
Why are they done?
Blood tests are done for lots of different reasons, including:
full blood counts (FBCs), which measure the number of red blood cells, white blood cells and platelets
liver function tests
tumour markers, where certain chemicals are released into your bloodstream - these can be measured to help diagnose a cancer and keep a check on how you’re responding to treatment.