A radioallergosorbent test is a blood test used to determine the substances a
subject is allergic to. This is different from a skin allergy test,
which determines allergy by the reaction of a person's skin to different
substances. Because there are other tests that help
with confirmation, results are best interpreted by a doctor. Since at least
2010, health organizations have recommended that the RAST be abandoned
as a diagnostic test for allergy in favor of more sensitive fluorescence
enzyme-labeled assays. Medical uses
The two most commonly used methods of confirming allergen sensitization are
skin testing and allergy blood testing. Both methods are recommended by the NIH
guidelines and have similar diagnostic value in terms of sensitivity and
specificity. Advantages of the allergy blood test
range from: excellent reproducibility across the full measuring range of the
calibration curve, it has very high specificity as it binds to allergen
specific IgE, and extremely sensitive too, when compared with skin prick
testing. In general, this method of blood testing vs skin-prick testing has
a major advantage: it is not always necessary to remove the patient from an
anthihistamine medication regimen, and if the skin conditions are so widespread
that allergy skin testing cannot be done. Allergy blood tests, such as
ImmunoCAP, are performed without procedure variations, and the results
are of excellent standardization. Adults and children of any age can take
an allergy blood test. For babies and very young children, a single needle
stick for allergy blood testing is often more gentle than several skin tests.
However, skin testing techniques have improved. Most skin testing does not
involve needles and typically skin testing results in minimal patient
discomfort. Drawback to RAST and ImmunoCAP
techniques do exist. Compared to skin testing, ImmunoCAP and other RAST
techniques take longer to perform and are less cost effective. Several studies
have also found these tests to be 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. In the NIH food guidelines issued in
December 2010 it was stated that “The predictive values associated with
clinical evidence of allergy for ImmunoCAP cannot be applied to other
test methods.” With over 4000 scientific articles using ImmunoCAP and showing its
clinical value, ImmunoCAP is perceived as “Gold standard” for in vitro IgE
testing Method
The RAST is 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: for
example, if a person exhibits a high level of IgE directed against pollen,
the test may indicate the person is allergic to pollen proteins. A person
who has outgrown an allergy may still have a positive IgE years after
exposure. The suspected allergen is bound to an
insoluble material and the patient's serum is added. 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 unbound
anti-human IgE antibodies are washed away. The amount of radioactivity is
proportional to the serum IgE for the allergen.
RASTs are often used to test for allergies when:
a physician advises against the discontinuation of medications that can
interfere with test results or cause medical complications;
a patient suffers from severe skin conditions such as widespread eczema or
psoriasis; 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.
Scale The RAST is scored on a scale from 0 to
6: History
The market-leading RAST methodology was invented and marketed in 1974 by
Pharmacia Diagnostics AB, Uppsala, Sweden, and the acronym RAST is actually
a brand name. In 1989, Pharmacia Diagnostics AB replaced it with a
superior test named the ImmunoCAP Specific IgE blood test, which
literature may also describe as: CAP RAST, CAP FEIA, and Pharmacia CAP. A
review of applicable quality assessment programs shows that this new test has
replaced the original RAST in approximately 80% of the world's
commercial clinical laboratories, where specific IgE testing is performed. The
newest version, the ImmunoCAP Specific IgE 0-100, is the only specific IgE
assay to receive FDA approval to quantitatively report to its detection
limit of 0.1kU/l. This clearance is based on the CLSI/NCCLS-17A Limits of
Detection and Limits of Quantitation, October 2004 guideline.
The guidelines for diagnosis and management of food allergy issues by the
National Institute of Health state that: In 2010 the United States National
Institute of Allergy and Infectious Diseases recommended that the RAST
measurements of specific immunoglobulin E for the diagnosis of allergy be
abandoned in favor of testing with more sensitive fluorescence enzyme-labeled
assays. See also
Prausnitz-Küstner test Skin allergy test
References External links
Allergies - Americal Academy of Allergy Asthma and Immunology
Allergy Blood Testing - Lab Tests Online Radioallergosorbent Test at the US
National Library of Medicine Medical Subject Headings