A RAST test or radioallergosorbent test is a way of testing a person's blood to see if they have any allergies. This test checks their blood for specific lgE antibodies to find out what substances they may be allergic to.
Allergies can be a mild annoyance or a life-threatening condition. Allergy tests allow a person to find out what substances they are allergic to so that they can plan ahead and avoid those allergens.
The RAST test is an alternative to the skin prick test. The skin prick test determines how a person's skin reacts to specific allergens.
In this article, we look at which allergies a RAST test can test for, the procedure, and interpreting the results. We also compare the effectiveness of RAST tests with that of skin tests.
The RAST test is a blood test that is used to see if an individual's blood contains antibodies for a specific substance, such as peanuts or pollen. These antibodies are called immunoglobulin E, or IgE antibodies.
If a person's blood contains lgE antibodies that are specific to a certain substance, it means they are allergic to that substance. These antibodies cause the rashes, itching, sneezing, and other symptoms that a person experiences when they come into contact with an allergen.
The name RAST was originally a brand name, but experts say it is now commonly and incorrectly used to describe any lab test for allergens.
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Different kinds of allergy tests are available, but the most commonly used is a skin or pinprick test.
In a skin test, small amounts of different allergens are placed on an individual's skin, usually with a pinprick. People who are allergic to these substances will develop itchy hives at these sites, while others without allergies will not.
The differences between the skin test and the RAST or ELISA blood tests are as follows:
- Speed of the procedure. Skin tests are faster than blood tests. A skin test can take place in the doctor's office, but in a RAST or ELISA test, the medical professional must send a blood sample to a laboratory for testing.
- Speed of results. Reactions to a skin test typically develop within 15 minutes, whereas it can take between a few days and 2 weeks to get the results of a RAST test.
- Accuracy. Skin tests may be more sensitive than blood tests, though both methods are considered accurate for diagnosing allergies. It may be more difficult to interpret accurately skin test results on people with darker skin, and skin tests may be affected by medications while blood tests are not.
- Safety. Although it is rare, a person can develop a serious reaction to an allergen used in a skin test. There is no risk of this with a blood test, such as RAST or ELISA.
- Cost. A skin test costs less than a RAST or ELISA test to process, which may be a consideration for some people.
In some cases, a person's doctor may recommend a blood test instead of a skin test. These cases can include:
- testing infants or young children, as blood tests require only one needle prick while skin tests require more
- avoiding the risk of a strong allergic reaction to a powerful allergen
- allowing individuals to stay on medication that could interfere with a skin test
- minimizing the risk of making an existing skin condition, such as psoriasis or eczema, worse
Blood tests like RAST and ELISA can test for a range of allergies, including food allergies, drug allergies, seasonal allergies, and pet allergies.
Along with diagnosing current allergies, blood tests can be used as part of the testing and treatment process that doctors use to test the progress of allergies in young children.
The presence and changes of lgE antibodies in the blood help a doctor to determine the progression of allergies, what allergy professionals call the 'allergic march,' that starts in infancy and progresses through childhood.
Doctors tended to avoid skin tests for infants, however. Researchers suggest that using blood testing procedures to diagnose allergies early in a person's life can provide the following benefits:
- the ability to start allergy intervention treatment earlier
- avoidance of dangerous reactions to food allergens in infants
- the potential to prevent the development of asthma
- a reduction in outbreaks of eczema
The RAST test procedure is fairly straightforward and does not require any preparation.
After a person speaks to their doctor, a medical professional will take a blood sample, usually from the individual's arm.
This blood is then sent to a laboratory, where it is subjected to a battery of tests that look for antibodies that have developed in response to specific allergens.
According to Food Allergy Research & Education (FARE), 50–60 percent of blood and skin prick tests will yield some "false positives" for food allergies, meaning the test will show that a person is allergic to something when they are not.
If a blood test finds that a person has antibodies for a specific allergen, it is likely that they have an allergy to the substance, but it is not definite. More tests may be needed.
For example, a RAST test may show that an individual is allergic to a food, such as chickpeas, only because it is in the same family as another food, for example, peanuts, that really do provoke an allergic response.
Also, the level of antibodies in the blood is not necessarily linked to the number of times an individual has been exposed to the allergen or how severe or mild those reactions may have been.
If an individual tests positive for a specific antibody, which is called a positive specific-IgE test, this result indicates that they have probably been exposed to the allergen. But it does not say for sure that the individual is allergic to the substance.
The likelihood of false positive results makes it that much more important for doctors to review RAST test outcomes in the light of an individual's overall medical history, particularly how much exposure they may have had to the allergen in question.
Allergies are very common, affecting more than 50 million people in the United States. Many people could, therefore, benefit from allergy testing to get proper treatment.
Allergies can affect the whole body. Key symptoms to watch out for include:
- abdominal pain
- itchy eyes
- nasal congestion
It is important to keep in mind that no allergy test can, by itself, determine whether or not a person has allergies and what those allergies are. A doctor will also take into consideration the individual's medical history and other factors.
Sometimes, different laboratories will use different versions or "brands" of a blood test. This means that test results from various versions of the test may not use the same scale or units of measurement. Hence, it is important to ask the doctor reviewing the test outcomes to put the information in context.
With 1 out of every 4 children in developed nations reportedly suffering from allergies, the RAST test can also play a role in reducing the suffering of these children and the progression of their allergic sensitivity.