A radioallergosorbent test (RAST) is a blood test using radioimmunoassay test to detect specific IgE antibodies, to determine the substances a patient 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. The results should be interpreted by a doctor.
The two most commonly used methods of confirming allergen sensitization are skin testing and allergy blood testing. Both methods offer 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 antihistamine 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.
Drawbacks 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 (CCDs).
With over 4000 scientific articles using ImmunoCAP and showing its clinical value, ImmunoCAP is perceived as “Gold standard” for in vitro IgE testing.
How a RAST works
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 (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
- a patient has such a high sensitivity level to suspected allergens that any administration of those allergens in the form of patch or skin prick testing might result in potentially serious side effects.
The RAST is scored on a scale from 0 to 6:
| RAST rating | IgE level (kU/L) | Result |
|---|---|---|
| 0 | < 0.35 | Undetectable allergen specific IgE |
| 1 | 0.35 – 0.69 | Low level of allergen specific IgE |
| 2 | 0.70 – 3.49 | Moderate level |
| 3 | 3.50 – 17.49 | High level |
| 4 | 17.50 – 49.99 | Very high level |
| 5 | 50.00 – 100.00 | Ultra high level |
| 6 | > 100.00 | Extremely high level |
How is a RAST performed?
RAST is performed on blood taken as a simple blood test. Blood is drawn from a vein (venepuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to fill with blood. A sterile needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding. Some people will experience bruising around the site.
Results are generally not available for a few days to a week and it is necessary to make another appointment with your doctor to discuss the results.
