Testing for Allergies
The Skin Tests: An allergy skin test may help diagnose the cause of the allergy. Skin tests are done on your arms or back using either the prick or intradermal method.
In the prick technique, a drop of an allergy-producing substance (allergen) is applied to the skin. The skin is then scratched or pricked. In the intradermal method, a tiny amount of allergen is injected into the skin. Skin tests for most allergens are safe, and the results are available in 20 to 30 minutes. Antihistamine and antidepressant medications affect skin tests and must be stopped before tests are begun. Some antihistamines can interfere with allergy skin tests for weeks. Check with your physician about stopping medications.
Skin tests for tobacco smoke allergy are not useful because tobacco smoke is an irritant, not an allergen.
Blood Allergy Tests
Allergies can also be diagnosed using a blood test. Examples include the radioallergosorbent test (RAST) or the ImmunoCap. They give information similar to the skin test. However, they are more expensive, may miss identifying some allergies, and also take longer to determine the results. The blood test can be used when a skin test would not be suitable. For example, it can be used when a person has a skin disease which does not allow skin tests to be done, or when taking an antihistamine or other medication that might affect skin test results.