Radioallergosorbent test (RAST)

RAST is a blood test using radioimmunoassay test to detect specific IgE antibodies, to determine the substances a subject is allergic to.

This is differs from a skin allergy test, which determines allergy by the reaction of a person’s skin to different substances.

Health organizations have recommended that the RAST test be abandoned as a diagnostic test for allergy in favor of more sensitive fluorescence enzyme-labeled assays.

Allergy blood tests have excellent reproducibility, a very high specificity as it binds to allergen specific IgE, and extremely sensitive too, when compared with skin prick testing.

With this method of blood testing it is not always necessary to remove the patient from an anthihistamine medication, and if eczema are so widespread that allergy skin testing cannot be done.

Adults and children of any age can take an allergy blood test.

Most skin testing today does not involve needles and typically skin testing results in minimal patient discomfort.

Compared to skin testing RAST techniques take longer to perform and are less cost effective.

Such tests are less sensitive than skin testing for the detection of clinically relevant allergies.

False positive results may be obtained due to cross-reactivity of homologous proteins or by cross-reactive carbohydrate determinants (CCDs).

A radioimmunoassay test to detect specific IgE antibodies to suspected or known allergens for the purpose of guiding a diagnosis about allergy.

IgE is the antibody associated with Type I allergic response.

The suspected allergen is bound to an insoluble material and the patient’s serum and if the serum contains antibodies to the allergen, those antibodies will bind to the allergen.

Radiolabeled anti-human IgE antibody is added where it binds to those IgE antibodies already bound to the insoluble material.

The amount of radioactivity is related to the serum IgE for the allergen.

RASTs used to test for allergies when: discontinuation of medications that can interfere with test results or cause medical complications; a patient suffers from severe skin conditions; or a patient has such a high sensitivity level to suspected allergens that any administration of those allergens might result in potentially serious side effects.

The RAST is scored on a scale from 0 to 6:

RAST rating IgE level (kU/L):

0 < 0.35 absent or undetectable allergen specific IGE.

1 0.35 – 0.69 low level of allergen specific IgE.

2 0.70 – 3.49 moderate level of allergen specific IgE.

3 3.50 – 17.49 high level of allergen specific IgE

4 17.50 – 49.99 very high level of allergen specific IgE.

5 50.00 – 100.00 ultra high level of allergen specific IgE.

6 > 100.00 extremely high level of allergen specific IgE.

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