When individuals are prescribed to take such medicines like an antihistamine, these specific drugs can cause an interference with the skin test that use commonly used to test for allergies. When this is the type of situation that occurs when a physician would like to find out what the specific allergens may be that are causing the patient to experience mild or severe allergic reactions, he will then generally decide that it is best to use an allergy blood test instead. There are two different types of allergy blood tests that a doctor may choose to perform. One is called RAST or a radioallergosorbent test, and the second is called ELISA or enzyme-linked immunosorbent assay. After small amounts of the patient’s blood are taken, it will then go through an analysis to search for antibodies that are known as IgE. When there are elevated levels of this antibody, it means that the patient does, in fact, have allergic reactions to the specific allergens that are tested. Allergy blood tests do cost quite a bit more money than the allergy skin test and they are not as effective. However, in certain situations this type of test is the best choice.