Allergy testing is done to determine what substance, or allergen, may trigger an allergic response in a person. Allergens cause reactions such as sneezing, runny nose (hay fever), and, sometimes, asthma attacks. If allergens are detected, a person can take steps to avoid or limit exposure to the substance or take allergy shots to control the allergic response.

Allergies are commonly detected through skin or blood tests. Either type of test can be used to determine what causes an allergic response by exposing a person to suspected allergens and seeing if a reaction occurs. Skin tests are usually done because they are rapid, reliable, and generally less expensive than blood testing.

Skin tests

One type of skin test is called a skin prick test. This test is done by placing a drop of a solution containing a possible allergen on the skin, then puncturing the skin under the drop with a needle to introduce the allergen solution into the top layer of the skin. If the skin reacts to the allergen with a red, raised itchy area (called a wheal), it usually means that the person is allergic to that allergen. This is called a positive reaction.

Another type of skin test is the intradermal test, which involves injecting a small amount of the allergen solution deeper into the skin. An intradermal allergy test may be done when a substance does not cause a reaction in the skin prick test but is still suspected as an allergen for that person. The intradermal test is more sensitive than the skin prick test and usually provides more consistent results.

A positive intradermal skin test means only that the person has antibodies to that allergen; it does not necessarily mean that the allergen causes a serious problem for the person. For example, many people who test positive to insect venom do not have an allergic reaction to insect stings. The person's history of symptoms is more important than the test results in deciding whether treatment for allergies is needed.

Another type of skin test, called a skin patch test, is used to detect a skin allergy called contact dermatitis. For a skin patch test, the allergen solution is placed on a pad that is then kept in contact with the skin for up to 2 days.

Skin testing commonly tests for allergies to several substances, including:

• Trees, shrubs, weeds, and grasses.
• Fungus.
• House dust, feathers, house mites, and pet dander.
• Foods.
• Drugs (such as penicillin).
• Insect venom.

Radioallergosorbent test (RAST)

Radioallergosorbent testing (RAST) may be done instead of or along with a skin test. RAST is used to measure the blood level of a type of antibody (called immunoglobulin E, or IgE) that the body may produce in response to particular allergens. IgE levels are often higher in people who have allergies or asthma. RAST may be used for people who cannot have skin tests, such as people who are taking certain medications (particularly some antidepressants) that can suppress skin test results.

RAST is done on a blood sample taken from a vein.

Allergy Tests
Results

Results of the skin tests will be available immediately after testing is done.

Results of RAST are usually available in about 7 days.