A radioallergosorbent test (RAST) is a blood test used to determine the substances a subject is allergic to. This is different from askin 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.
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 (fluorenzymeimmunoassay), 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.
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 (in-vitro, out of body) vs skin-prick testing (in-vivo, in body) has a major advantage: it is not always necessary to remove the patient from an anthihistamine medication regimen, and if the skin conditions (such as eczema) 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 tocross-reactivity of homologous proteins or by cross-reactive carbohydrate determinants (CCDs).
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
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 theantibody 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 (or pollen-like) 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.
There are a number of different blood tests for IgE available:
- Total IgE: this measures all IgE in the blood; this is not usually a helpful test, as a number of conditions cause IgE to be high (including eczema) and IgE levels do not necessarily relate to food allergy.
- Specific IgE (previously known as a RAST): this measures the amount of IgE to a specific food allergen, (a protein that can cause a reaction), such as peanut or egg. However, the test can give an elevated result without the patient having any symptoms (this is called “sensitisation”; it affects one-third of the population) and the elevated IgE is harmless. When an elevated result is seen in conjunction with symptoms to that allergen we can term the condition an “allergy” and measures should be taken. Therefore, specific IgE testing should only be requested against an allergen against which the patient has complained of symptoms and random testing is not recommended.
- Component resolved IgE testing: also known as component resolved diagnosis (CRD) – this is a very recent development in specific IgE testing, whereby the laboratory can detect IgE to specific pieces of an allergen. Early evidence demonstrates that for some food allergens, such as peanut, a positive CRD to a particular part of the peanut protein may be more likely to indicate severe rather than mild allergy. Certain tests are therefore now available in specialist clinics using CRD methods.
- IgE panels / point of care testing: some IgE blood-testing kits have now become available for the patient to use at home or in a pharmacy. In general, these tests are not specific to a particular food and are therefore not easy to interpret; both false positive and false negative results are possible, and the presence of IgE in a test does not necessarily mean that the substance is actually causing the patient’s symptoms. For these reasons, these tests are not recommended.
Blood test results can take a few weeks to come back, and the amount of IgE antibodies measured in the blood needs to be above a particular level before an allergy is suspected or confirmed. The level of IgE antibodies recorded does not indicate how serious the allergy is, merely if the patient is possibly allergic or not. Interpreting the results requires care and experience and should always be done by a trained clinician in conjunction with the clinical history.
How is it used?
The allergen-specific IgE antibody test is a blood test used to help diagnose an allergy to a specific substance or substances for a person who presents with acute or chronic allergy-like symptoms. This is especially true if symptoms are recurrent and appear to be tied to triggers, such as exposures to particular foods or environments, and if other family members are known to have allergies.
Other types of allergy tests may be performed by exposing a person to different substances under careful medical supervision. The usefulness of these tests, however, can be affected by skin conditions, such as significant dermatitis or eczema, and by medications, such as histamines and some anti-depressants. With some tests there is also the potential for severe reactions, including one that may be life-threatening such as anaphylaxis. In these cases, the allergen-specific IgE antibody test may be ordered as an alternative, as it is performed on a blood sample and does not have an effect on the person being tested.
The allergen-specific IgE antibody test may also be performed to monitor immunotherapy (desensitization) or to see if a child has outgrown an allergy. It can only be used in a general way, however, as the level of IgE present does not correlate to the severity of an allergic reaction and someone who has outgrown an allergy may have a positive IgE for many years afterward.
When is it ordered?
One or more allergen-specific IgE antibody tests are usually ordered when a person has signs or symptoms that suggest anallergy to one or more substances. Signs and symptoms may include:
- Red itchy eyes
- Coughing, nasal congestion, sneezing
- Itching and tingling in the mouth
- Abdominal pain or vomiting and diarrhea
A test may also be ordered occasionally to help evaluate the effectiveness of immunotherapy or to determine whether a child has outgrown an allergy.
What does the test result mean?
Results of allergy blood testing must be interpreted with care. Even if an IgE test is negative, there is still a small chance that a person does have an allergy. Similarly, if the specific IgE test is positive, a person may or may not ever have an actual physical allergic reaction when exposed to that substance.
Negative results indicate that a person probably does not have a “true allergy,” an IgE-mediated response to the specific allergens tested.
An elevated allergen-specific IgE result indicates that the person tested likely has an allergy. However, the amount of specific IgE present does not necessarily predict the potential severity of a reaction. A person’s clinical history and additional medically-supervised allergy tests may be necessary to confirm an allergy diagnosis.