Allergy Testing

Allergy tests are used to find the substances that are causing an allergic reaction. An allergicreactionis an immune system response to a substance known as a trigger.

Who is a candidate for the procedure?

The healthcare provider may perform or order allergy tests onpeople with suspected allergies. Skin testing is usually not done on childrenyounger than age 3. It may be done onolder individuals to determine the triggers that are causing allergic reactions in the person.Before the procedure, aperson should not take medicines that could block skin reactions. For example,an antihistamine may block a skin reaction.

How is the procedure performed?

There are many methods of allergy testing.

Skin tests are used to identify specific problem allergens. The healthcareprovider will select skin tests based on a person’s particular history. Testsolutions are made from extracts of various inhaled, ingested, or injectedsubstances. Test solutions available include extracts from tree, grass, andweed pollens; dust mites; animal dander; insect venoms; foods; and penicillinand penicillin derivatives.

For the prick skin test, a small amount of the substance thought tocause the allergic reaction is placed on the skin. This substance is called anallergen. The skin is then pricked or scratched. This allows the substance toget under the skin’s surface. If the person is allergic, the skin will usuallyget red and swell within about 20 minutes.

Another skin test is called an intradermal test. A small amount ofthe allergen is injected beneath the skin. This more sensitive test is oftenused when the prick test has produced a negative or uncertain result inreaction to suspected inhaled allergens.

An elimination diet test may be used to diagnose food allergies. For thistest, a person goes several weeks without eating any of the foods that may becausing the problem. Foods are returned to the diet one at a time. If allergicsymptoms come back after eating a certain food, that food is probably causingthe problem. If a specific food is suspected, it can be given to the person. Ifa reaction occurs, this food is the likely cause.

The best way to test for foodallergies is by using an oral food challenge. This test can be usedeven with smallchildren. The suspected food is removed from the diet. After 4 to 5 days, thefood should be eaten on an empty stomach. This is the best time to watch for areaction. Parents can keep a food diary for their children. This can helpidentifythe foods causing the problems.

Blood tests measure antibodies to a particular allergenin blood. One blood test for allergies is called a radioallergosorbent test, orRAST. In a true allergic reaction, substances called IgE antibodies appear inthe bloodstream. The body makes IgE antibodies to attack bacteria and otherharmful substances. RAST measures allergen-specific IgE. Compared to skintests, RAST has the disadvantage of limited allergen selection and reducedsensitivity.

A newer version of a blood test is called the Immunocap. A recentstudy indicated that the Immunocap test was significantly more accurate thanthe older blood tests. Another study compared the accuracy of skin testing toblood testing for cat allergy. This study reported that skin testing and bloodtesting were about equal in accuracy.

Other blood tests, such as an eosinophil count, may be used tosupport an allergy diagnosis. Provocative testing is the direct application ofan allergen to the eyes, nose, lungs, or gastrointestinal tract by oraladministration. It may be helpful in cases where a person has had a largenumber of positive skin tests. Provocative testing is the only way to check forallergies to food additives. This type of testing may produce a severe allergicreaction and is used rarely.

What happens right after the procedure?

After a short wait in the office to watch for any allergic reactions, the person can usually gohome.

What happens later at home?

If there is no response to the allergen, the test is negative.This indicates that the person is not allergic to the substances tested. If atest is positive, the person has an allergy to that substance. That substanceshould be avoided. Occasionally, a test may be inaccurate. For example, aperson might react to a substance during testing and never have a problem withit during daily life.

What are the potential complications after the procedure?

When a person is exposed to something to which he or she isallergic, he or she can have a reaction. Skin testing and food testing cancause severe allergic reactions, including anaphylaxis. Emergency equipment and access to theemergency medical systemshould be readily available. Any new or worsening symptoms should be reportedto the healthcare provider.