While numerous tests can be conducted to determine if a patient has allergies, these procedures fall into two main groups: skin testing and blood testing. Skin allergy tests are more commonly conducted than blood allergy tests. Of the skin tests, three procedures can be performed: prick, intradermal, or patch.   A prick test is what most people associate with an allergy test, as it allows for multiple allergens to be sampled at once in a grid formation. Allergen sample solutions will be placed on the back or forearm, and a few pricks or minor scrapes will be made to allow the solution to enter the wound. If a wheal, or "hive," appears, this is considered a positive result confirming allergies. On the other hand, a prick test may yield negative results even if an allergy exists; in this case, an intradermal test will likely be performed. In this instance, a small amount of the allergen solution will be injected just under the skin in the dermal (second) layer.   Again, if a wheal appears, the test is positive. Finally, contact dermatitis, a particular rash caused by contact with an irritant or an allergic reaction, can be tried with a skin patch. This patch containing allergens is kept on the skin for twenty-four to seventy-two hours in an attempt to provoke a rash. Allergies caused by food, medications, airborne allergens like pollen, or insect stings are best confirmed with skin tests.   Alternatively, a blood test may be performed, though it is less sensitive than a skin test. Choosing a blood test over a skin test may be due to a patient's skin condition. For example, the appearance of eczema hives may skew the results of a skin test.   Further, if patients are taking continuous antihistamine medication, the effects of the drug will suppress allergic reactions and could result in a false negative. Blood tests differ from skin tests in that the level of immunoglobulin is measured, a particular type of antibody made by the immune system to fight off an allergen.