Definition

Allergy tests are any of several tests used to determine the substances to which a person is allergic.

Blood Tests

Blood tests can be done to measure the amount of immunoglobulin (Ig) E antibodies to a specific allergen in the blood. This test may be used when skin testing is not helpful or cannot be done.

Other blood tests include:
• Absolute eosinophil count
• Total IgE level
• Serum immunoglobulin electrophoresis

Provocation

Provocation (challenge) testing involves exposing a person to a suspected allergen under controlled circumstances. This may be done in the diet or by breathing in the suspected allergen. This type of test may provoke severe allergic reactions. Challenge testing should only be done by a doctor.

How to prepare for the test

Before any allergy testing, the health care provider will ask for a very detailed medical history. This may include questions about such things as illnesses, emotional and social conditions, work, entertainment, lifestyle, foods, and eating habits.

If skin testing will be performed, you should NOT take antihistamines before the test. This may lead to a false-negative result, falsely reassuring you that a substance is unlikely to cause a severe allergic reaction. Your doctor will tell you which medicines to avoid and when to stop taking them before the test

How the test will feel

Skin tests may cause very mild discomfort when the skin is pricked. Itching may occur if you have a positive reaction to the allergen.

Why the test is performed

Allergy tests are done to determine the specific substances that cause an allergic reaction in a person.

Your doctor may order allergy tests if you have:

• Allergic rhinitis and asthma symptoms that are not easily controlled with medications
• Angioedema and hives
• Food allergies
• Contact dermatitis
• Penicillin allergy*

*NOTE: Allergies to penicillin and closely related medications are the only drug allergies that can be tested using skin tests. Skin tests for allergies to other drugs can be dangerous.
The prick skin test may also be used to diagnose food allergies. Intradermal tests are not used to test for food allergies because of high false positive results and the danger of causing a severe allergic reaction.

Normal Values

In a nonallergic person, allergy tests should be negative (no response to the allergen).

What abnormal results mean

A positive result means you reacted to a specific substance. Often, but not always, a positive result means the symptoms that you are having are due to exposure to the substance in question. In general, a stronger response means you are more sensitive to the substance.

People can have a positive response with allergy skin testing, but not have any problems with the specific substance in every day life.

The skin tests are generally reliable. However, if the dose of allergen is excessive, a positive reaction will occur even in persons who are not allergic.

What the risks are

Risks related to skin and food allergy tests may include:

• Allergic reaction
• Life-threatening anaphylactic reaction

Special considerations

The accuracy of allergy testing varies quite a bit. Even the same test performed at different times on a person may give different results. A person may react to a substance during testing, but never react during normal exposure. Rarely, a person may also have a negative allergy test and still be allergic to the substance.

References

Bernstein IL, Li JT, Bernstein DI, Hamilton R, et al. American Academy of Allergy, Asthma and Immunology; American College of Allergy, Asthma and Immunology. Allergy diagnostic testing: an updated practice parameter. Ann Allergy Asthma Immunol. 2008 Mar;100(3 Suppl 3):S1-148.

Demoly P, Bousquet J, Romano A. In vivo methods for the study of allergy. In: Adkinson NF Jr, ed. Middleton’s Allergy: Principles and Practice. 7th ed. Philadelphia, Pa: Mosby Elsevier; 2008:chap 71.

Gober MD, DeCapite TJ, Gaspari AA. Contact dermatitis. In: Adkinson NF Jr, ed. Middleton’s Allergy: Principles and Practice. 7th ed. Philadelphia, Pa: Mosby Elsevier; 2008:chap 63.