Allergy testing can tell you what you are allergic to. Testing may reveal allergens that you didn't even realize were causing you problems. Furthermore, testing is necessary if you wish to start immunotherapy (allergy shots).
The allergist will ask questions about your medical history to determine whether allergies run in your family. The allergist may ask detailed questions about your symptoms, what you did to treat those symptoms, and whether it worked. Once non-allergic conditions are evaluated and allergy is suspected, your allergist will perform a diagnostic allergy test.
Skin prick or scratch test
When most people go to the allergist for the first time, they want to know right away -- "What am I allergic to?" Fortunately, skin testing can usually be done on your first visit, and you may get immediate answers to your questions. However, some medications may affect the accuracy of the test, such as antihistamines and antidepressants. If you are taking any prescription medications, ask your primary care physician and allergist how to prepare for the allergy tests.
The skin prick or scratch test is the most common, reliable test for most allergies. The procedure is fairly painless. A small needle or plastic device is used to lightly prick or scratch your back or forearm with a tiny amount of allergen. After 15 - 20 minutes, your allergist will be able to interpret the results by examining each spot where allergens were scratched or pricked into your skin. The spots where you are allergic will become red and swollen, and the others will remain normal.
The intradermal test is typically done when the skin prick or scratch test results are negative. It is similar to the prick or scratch test but involves injecting a small amount of allergen under the skin using a needle. The test is also more sensitive, exposing the skin to more allergen than the prick or scratch test. The test is usually done only to see if you are allergic to medicines. It should never be done to test for food allergies, as it may cause an allergic reaction.
Reactions to skin testing should clear up quickly. Because skin testing involves the injection of allergens under the skin, there is a small risk of anaphylaxis. For this reason, allergy skin testing should only be performed in a medical setting, with access to emergency treatment.
The blood test or RAST (radioallergosorbent) test measures the levels of the allergy antibody IgE to a specific allergen in your blood. If you are allergic to a substance, the IgE levels against that substance are usually high. The blood test may be used if you have existing skin problems like eczema, if you're on medications that are long-acting or you cannot stop taking, if you have a history of anaphylaxis, or if you prefer not to have a skin test. A drawback of the blood test is that it may take several days to get the results. Also, other conditions are associated with elevated IgE levels (such as HIV, skin diseases, and parasitic diseases), so the results are not always accurate and need to be compared to your allergy symptoms and medical history.
To find out whether you have an allergy, if your allergist is not certain after testing your skin or blood, you may need a challenge test. These tests are usually done only for food or drug allergies. There are two types of challenge test: open and blinded. In an open challenge, you gradually swallow increasing amounts of a suspected allergen while you are monitored for allergy symptoms. In a blinded challenge, you swallow a very small amount of the suspected allergen (such as milk or antibiotic), usually in a capsule. Real capsules may are alternated with placebo capsules. That way, your health care provider and you do not know what you are taking. This helps eliminate any uncertainty.
Due to the risk of a severe allergic reaction like anaphylaxis, challenge tests are done in a clinical setting and are only performed when absolutely necessary.
Snapshot of a Moving Picture
Most people think of specific allergies in black and white terms -- something you either have or you don't. However, being allergic to something is a continuum -- and that continuum changes over time. Most (but not all!) food allergies get better over time. Most airborne allergies get more common as children get older. Some allergies peak before puberty and then disappear. Others don't even begin until puberty is over.
Furthermore, test results must be interpreted differently at different ages. Under age 1, a positive test is usually a true allergy, but a negative test does not tell you anything. In children over 3 or 4, the reverse tends to be true -- a negative test means the child is probably not allergic to that substance, whereas a positive test does not necessarily mean that the substance causes symptoms for the child.
Most people who do get tested for allergies have a single round of skin testing. This can provide a valuable snapshot of allergies at a single moment in time.
Alan Greene, M.D., F.A.A.P.