A blood test involves taking a small sample of blood from your child, from the back of the hand, inside of the elbow or the wrist.
The area where the blood is being taken from may be anaesthetised (numbed) first with a special spray.
Most blood tests only take a few minutes to complete.
The procedure shouldn't hurt much and our specially trained staff will do all they can to make your child as comfortable as possible.
The Great Ormond Street Hospital website has some useful tips and advice on what to do if your child is scared of needles.