Introduction to RAST (serum specific IgE)
The radioallergosorbent testing (RAST test) detects the IgE allergen in the blood. They are used when skin prick tests (preferred allergy tests) are not appropriate or available. Remember that hypersensitivity tests can’t be utilized all alone and must be deciphered related to side effects and clinical history. Government health care discounts are accessible to RAST.
Important note: unproven allergy tests
There are many methods claiming to test for sensitivity including cytotoxic food test, kinesiology, Vega test, electro skin test, pulse test, reflexology, and hair analysis. These tests have not been scientifically validated and may lead you to take unnecessary and expensive avoidance strategies (in the case of some dietary changes). A Medicare discount is not available in Australia for these tests and these methods are not recommended.
What is a RAST test?
The RAST test is a blood test used to find out if an individual’s blood contains antibodies to a specific substance, such as peanuts or pollen. These are called immunoglobulin E or IgE antibodies. If a person’s blood contains antibodies to lgE specific to a specific substance, this means that they are allergic to that substance. These antibodies cause rashes, itching, sneezing, and other symptoms that a person experiences when they come into contact with an allergen.
RAST was originally a brand name, but experts say it is now used commonly and incorrectly to describe any allergen lab tests. According to the guidelines for the diagnosis and management of food allergies in the united states, the original RAST testing method is now outdated. Instead of the RAST tests, your doctor will likely order a different blood test called ELISA, which is an enzyme-linked immune absorption test.
When would you need a RAST test?
- Your doctor may order a RAST test for severe allergies, asthma, or hay fever, where the precipitate (known as the antigen) is unclear.
- A subset of antibodies called IgE antibodies is closely related to the abnormal immune system response to allergic reactions.
- Among these IgE antibodies, the allergen-specific IgE antibody numbers can be measured by means of the RAST test.
Risks of a RAST test
The risks associated with RAST are limited to the risks of having a blood test, and include:
- Excessive bleeding (most likely if you have a bleeding or clotting disorder)
- Fainting or feeling dizzy (usually resolves quickly)
- Hematoma (accumulation of blood under the skin)
- Infection (a slight risk any time the skin is broken)
- Multiple punctures to locate the veins (venipuncture is easier on some people than others)
RAST test procedure
Although RAST is not as sensitive as the intradermal skin test, it has several advantages when used in conjunction with or as an alternative to traditional testing procedures.
- Safe: RAST results are not influenced by allergy symptoms or are confirmed by medications used to treat allergy symptoms. Skin testing may aggravate patients with severe symptoms of asthma, urticaria, or atopic dermatitis.
- These skin tests cannot be performed on patients who are using antihistamines because these drugs will dampen the results. Tingling or scratching in skin tests may cause discomfort and may itch and swell. Since RAST is performed outside the patient, it is not affected by disease or medication.
- Especially in younger children, the allergy skin test is painful and these patients rarely feel so anxious that an allergy skin test cannot be performed. In such cases, RAST may be a good alternative method.
- No specialist required: While the skin prick test requires a health professional to perform the procedure and interpret the results, RAST is straightforward and does not require any expertise, experience, or special reagents for the medical practitioner performing the test.
- No early preparation: RAST can be performed without avoiding medication and antihistamine medication, a few days before the test. In contrast, a skin prick test with these drugs 7 days prior to the test would interfere with the test results. There are also no restrictions on the use of creams and moisturizers that may cause other allergic reactions in the skin prick test because of the allergen extracts they contain.
Limitations of RAST
- It is less sensitive than allergy skin tests at detecting small amounts of clinically relevant IgE antibodies. Several studies have shown that false-negative results may be as high as 50% because the endpoint sensitivity is so high.
- Limited availability of RAST allergens
- Poor quality control of RAST and its use in many commercial laboratories
- RAST is expensive.
What do the results of a RAST mean?
The RAST test is performed against a group of allergens, such as a food allergy screen, or against specific allergens, such as house dust mites. If the test is negative, it is possible that you are not able to form an allergic reaction to this allergen, and any symptoms you experience may not be due to an allergic reaction to the substance.
If the test is positive for a specific allergen, such as house dust mites, then this indicates that you are sensitive to that specific allergen. This means that you have the ability to respond to an allergen when you are exposed to these allergens. This does not necessarily mean that you have a clinical allergic reaction to the substance, nor does it mean that any reactions will be severe. RAST results should be interpreted in conjunction with your clinical history and pattern of allergic reactions.