Overview of Allergy Tests
Allergy testing is used to help determine the cause for allergic reactions. The results of allergy tests can help patients and physicians develop a treatment plan to effectively manage allergy symptoms. Allergy testing may include a medical history, a physical examination, allergy skin tests, allergy blood tests, and food allergy testing.
An allergy is an abnormal immune system response that occurs as a result of exposure to certain substances (called allergens). An allergic reaction occurs when an allergen-specific antibody called immunoglobulin E (IgE), which is produced by the immune system and binds to cells in the body (mast cells), comes into contact with the specific allergen for which it was produced.
Allergens cause the immune system, specifically white blood cells, to produce IgE antibodies that attach themselves to mast cells or basophils. The allergen attaches to specific IgE antibodies on the surface of the mast cell, and when the allergen and the IgE antibodies combine, the release of histamine (substance that dilates blood vessels) and other chemicals is triggered, causing allergy symptoms.
Allergy symptoms include the following:
- Eczema (atopic dermatitis, contact dermatitis)
- Hives (urticaria) and or swelling (angioedema)
- Itchy, watery eyes (allergic conjunctivitis)
- Runny nose, sneezing
- Shortness of breath, chest tightness, wheezing
Allergists/immunologists are physicians specially trained to diagnose and treat allergies. If an allergy is suspected, the physician may ask the patient to keep a record of when, where, and under what circumstances the symptoms occur. To help diagnose an allergy, the allergist/immunologist takes a medical and family history (allergies often run in families) and performs a physical examination and diagnostic tests. A medical history includes information about symptoms (e.g., where and when they occur) and information about the patient's home, work, and school environment.
During physical examination, the allergist/immunologist examines the ears, eyes, nose, throat, skin, and lungs. In some cases, a pulmonary function test is performed to determine how well the patient is able to expel air from the lungs.
Types of Allergy Testing
Skin tests often are used to diagnose allergies. These tests involve little discomfort and take about 30 minutes to perform. An allergist/immunologist interprets the results of the test in conjunction with the patient's history and uses these results to determine the best course of treatment. Treatment may include medications and allergy shots (immunotherapy).
Prior to allergy skin tests, a positive histamine control test and a negative saline control test may be performed. A positive control test is used to determine if the patient reacts to histamine. If the patient does not immediately react to histamine, the results of allergy skin tests can be difficult to interpret. A negative control test involves applying a saline solution that does not include any allergens. Patients who react to this solution may have skin that is too sensitive to allow correct interpretation of allergy skin tests.
There are two types of skin tests, prick tests and intradermal (i.e., under the skin) tests. Prick tests involve placing small drops of common allergens on the skin (usually on the forearms or back) and then lightly pricking the skin through the drop with a small needle. Intradermal tests involve injecting a small amount of allergen into the outer layer of skin. When a patient is allergic to a substance, redness, itching, and swelling develop at the site of the test within 20 minutes. After the test, a mild cortisone cream may be applied to reduce itching.
Patch tests can be used to diagnose contact dermatitis. In this test, the allergist/immunologist places a small amount of allergen on the skin (usually on the back), covers the area with a bandage, and checks for a reaction after 4872 hours. Patients who are allergic to the substance develop a rash, or even blisters, on the skin.
Certain medications (e.g., antihistamines, antidepressants) and skin conditions (e.g., eczema) can interfere with allergy skin tests. Patients who must continue to take these medications and patients who have a severe skin condition may require a blood test to diagnose allergies.
Allergy blood tests involve taking a blood sample, adding an allergen to the sample, and measuring the amount of immunoglobulin E (IgE) antibodies produced in response to the allergen. Types of allergy blood tests include the following:
- Enzyme-linked immunosorbent assay (ELISA)
- In vitro basophil histamine release assay
- Radioallergosorbent test (RAST)
Allergy blood tests, which are less sensitive and more expensive than skin tests, are usually reserved for rare cases when allergy skin tests may not be accurate (e.g., when the patient has sensitive skin that reacts to a saline prick test or has a skin condition, such as hives or eczema, that prevents an adequate field for skin tests).
In addition to allergy skin tests and allergy blood tests, patients with a suspected food allergy may undergo food allergy tests. Food allergy testing often begins with keeping a food diary, which is a detailed list of all foods, the date and time they were eaten, and any symptoms that occurred.
When a single food allergy is suspected, the patient may be advised to eliminate the food from the diet and then, if symptoms are relieved, add the food back to the diet to determine if an allergic reaction occurs. This allergy test is not used in patients with a history of severe allergic reaction (anaphylaxis).
If the results of these food allergy tests are inconclusive, the allergist/immunologist may perform a "blinded" food allergy test and/or a challenge test. These tests usually are performed in a physician's office or in the hospital, and they are closely supervised.
They involve feeding the patient either the suspected food, or a neutral food (called a placebo), and then monitoring the patient for an allergic reaction. Neither the patient nor the physician knows whether the suspected food or the placebo is being given to the patient. The results of these allergy tests are very reliable.