Allergy Testing

Sort out your allergies with allergy testing. Learn about the skin tests and when a blood test is used.

By Louis Neipris, MD, Staff Writer

It's a simple cause and effect. Every time you sweep or vacuum you sneeze. That's a dust allergy, right? So why do you need allergy testing? The truth is that allergic conditions are often not that straightforward. And even that example doesn't tell the real story. Why? Because it's not just dust, but millions of particles from one or more sources (dust mites, pet dander or pollen) to which you may be allergic. Allergy testing can sort out what exactly is causing your symptoms. With the results of the tests, you know exactly what to avoid.

What are the allergy tests?
The two main types of allergy tests are skin testing and a blood test also called RAST (radioallergosorbent test).

Why would I need allergy testing?
If you have allergy symptoms such as sneezing, itchy eyes, nasal congestion, swelling and wheezing, an allergy test can be done to identify which specific substances (allergens) trigger an allergic reaction.

Skin tests are done to diagnose:

If you have a skin condition or take certain medications that may interfere with skin testing results, doctors may use a blood test to pinpoint allergies instead.

Which common allergens can be tested for?

How are skin tests done?
When there are several suspected allergens, a "grid" format may be used, so it's clear where each allergen is being placed. The back or the inner forearm may be used for the test, depending on the number of allergens being tested. A small dose of the suspected allergen is "scratched" on the surface of the skin (skin prick test) or injected just under the skin (intradermal test).

What do the results mean?
Within 15 minutes of applying the allergen, if you are allergic to what was introduced to the skin, you will have a positive reaction. For example, if you are allergic to cat dander, in the site where the cat dander allergen is applied there will be a small bump (or hive) and the skin around the bump may turn red. The site may feel itchy. The bump and redness go away within 30 minutes. Sometimes the results of a skin test are not as clear, but may still point to allergies.

Could I have a serious reaction to allergens tested?
Skin testing for allergy diagnosis is done in the allergist's office. Here you are monitored for a rare, serious reaction called anaphylaxis (facial swelling, trouble breathing).

What is a challenge test?
If you are allergic to food or medication, the allergist may do a challenge test. For this test, you inhale or swallow a very small amount of the suspected allergen, such as milk or an antibiotic. If there is no reaction, the dose may be slowly increased. Because challenge tests may cause severe allergic reactions, they are only done when really needed, and must be closely monitored by an allergist.

What is the blood test?
Blood is drawn from a vein and then sent for testing for the presence of allergen-specific antibodies (IgE antibodies). The skin test, though, is more sensitive and cheaper than the blood test and gives immediate results.

Who does allergy testing?
Allergy testing is usually performed by an allergist/immunologist or other physician who has additional training in diagnosing and treating allergic conditions.