Step 6: Allergy testing and diagnosis

Allergy testing can let you know for certain which allergens are affecting you. 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 ruled out 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.

Intradermal test

The intradermal test is done when the skin prick or scratch test results are unclear. It is similar to the prick or scratch test, but involves injecting a small amount of allergen under the skin using a needle.

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.

Blood test

The blood test or RAST (radioallergosorbent) test measures the levels of the allergy antibody IgE that is produced when your blood is mixed with a series of allergens in a laboratory. If you are allergic to a substance, the IgE levels may increase in the blood sample. 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. Some drawbacks of the blood test are the cost and the time required to wait for 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 definitive and need to be compared to your allergy symptoms and medical history.

Challenge test

To confirm a food or drug allergy after a skin or blood test result is positive, your allergist may perform a challenge test. For the challenge test, you swallow a very small amount of the suspected allergen (such as milk or antibiotic), usually in a capsule. Real capsules may be alternated with placebo capsules. If there is no reaction, your allergist gradually gives you more until a reaction is noted. 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. A study published in the January 2002 issue of the American Journal of Respiratory and Critical Care Medicine emphasizes that the truth is much more complex. 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, but this just "scratches the surface" of a child's long-term allergy story. Alan Greene, M.D., F.A.A.P.

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