There are several ways of doing this which include blood tests, skin prick tests and patch testing. A blood test involves the taking of a small sample of blood which is examined under a microscope. The aim is to detect any changes which indicate a particular skin condition, e.g. high levels of antibodies. Patch testing and a skin prick test involve the application of small samples of potential allergens (substances which cause an allergic reaction) onto the skin. These are left there for a period of 24 to 48 hours. They are then removed and the skin checked for signs of an allergic reaction.