An allergen-specific immunoglobulin E (IgE) test, commonly often referred to as radioallergosorbent test (RAST®) is a type of blood test that can help determine if a patient who experiences allergy symptoms, such as runny nose, watery eyes and hives, is allergic to particular substances called allergens.
The in vitro test (performed outside of the body, in a laboratory setting) exposes a blood sample to suspected allergens (like dust mites, pollen or animal dander) to determine whether the patient has developed allergen-specific immunoglobulin E (IgE) antibodies. Antibodies are proteins that recognize and bind to specific antigens (foreign substances in the body).
The test is used to diagnose Type I allergic reactions. Type I allergic reactions involve IgE, which is specific for a particular drug, antigen or other allergen that triggers the allergic reaction. The allergen binds to the immunoglobulin on specific immune cells known as basophils and mast cells. This binding results in the release of chemicals that cause inflammation in the body within 30 minutes of exposure to the allergen. Inflammatory chemicals including histamine, serotonin, proteases, bradykinin generating factor, chemotactic factors (molecules that stimulate the immune response), leukotrienes, prostaglandins and thromboxanes are released. These chemical mediators cause symptoms associated with allergies such as hives, runny nose, watery eyes, sneezing, wheezing or itching. Type I allergic reactions are often triggered by food (like peanuts, shellfish and milk), drugs (like penicillin), inhaled allergens (like dust, pollen and pet dander), latex and insect venom (bees, wasps, etc.).
The test may also be conducted to monitor immunotherapy (allergy shots) or to determine whether an individual has outgrown an allergy.
The antibody test is less accurate than a skin test. However, if the patient has a severe skin disease (like eczema or psoriasis) that is present on large areas of skin on the arms or back, a skin test may not be possible. This is because the skin test may only be performed on the arms and back, and there may not be enough unaffected skin to perform a conclusive test. For these patients, an allergen-specific IgE test is the preferred diagnostic method.
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Pharmacia Diagnostics AB developed the allergen-specific IgE test, which was first marketed as RAST® in 1974. In 1989, Pharmacia Diagnostics AB replaced it with a superior test called the ImmunoCAP® specific IgE blood test, which is more sensitive and specific. The newer test is a cellulose polymer in a plastic reserve, which has a higher binding capacity of allergy proteins than the RAST® test. According to medical literature, the new test has replaced about 80% of the old RAST® tests in laboratories worldwide.
The newest version of ImmunoCap® is the only specific IgE assay to receive approval by the U.S. Food and Drug Administration (FDA).
how it is performed
The allergen-specific IgE antibody test is an in vitro test, which means it is conducted outside of the body in a laboratory setting.
During the procedure, a sample of blood is taken from the patient.
The blood is then sent to a laboratory that performs specific IgE blood tests. Currently, more than 550 allergens are available for determinations.
An allergen-antigen complex is bound to an allergosorbent (paper disk) and the patient's blood is added. If the blood contains antibodies to the specific antigen, it will bind to the "tagged" immunoglobulins.
The patient will receive the test results in 7-14 days.
A qualified healthcare provider will interpret the results of the test. In general, the sensitivity of these tests range from 50-90%, with the average being about 70-75%.
Elevated levels of antibodies indicate an allergy. The amount of antibodies present is not proportionate to the severity of the allergy.
Patients who have allergies may be advised to avoid the allergen or they may be prescribed allergy medications, such as antihistamines (like diphenhydramine) or anti-inflammatories (like prednisone). However, treatment depends on the specific allergen and the severity of the allergy. Therefore, patients should consult their healthcare providers if they test positive.
Patients who test negative will have extremely low or nonexistent levels of antibodies. Negative results indicate that the patient does not have an allergy.
Tests results may not always be definitive because of accuracy limitations of individual methods. Results can be misleading if an inadequate amount of the allergen was used, or if the allergen was not standardized. In addition, researchers have not agreed on a definitive antibody level that indicates a positive result. If the results are inconclusive, a skin test may be recommended. During a skin test, the skin is exposed to different allergy-causing substances (allergens) and then observed for an allergic reaction.
There are no medical risks associated with the allergen-specific IgE antibody test. Slight bruising may occur at the site where blood was taken.
Unlike skin tests, there is no risk of a serious allergic reaction occurring during the antibody test because the patient's body is not exposed to the allergen.