Also called: Allergy Screen, Radioallergosorbent Test, Allergen-Specific IgE Antibody Test
Marc J. Sicklick, M.D., FAAAAI, FACAAI
The radioallergosorbent test (RAST) and related tests (e.g. RASP, FAST) are blood tests performed to screen for specific triggers of an allergy. The RAST is one of several types of allergy tests given to identify one or more allergens.
By identifying an allergy’s trigger, steps can be taken to limit exposure to those triggers (allergens) or to reduce symptoms through immunotherapy. A RAST requires blood to be drawn from a patient and sent to a laboratory for analysis. A physician will look for specific antibodies to determine what may be causing an allergy.
The accuracy of the RAST is comparable to skin tests, which are more commonly given to identify an allergen, but the cost is higher and results take longer to receive. A RAST is used instead of a skin test when:
- Certain medications can suppress or interfere with skin test results (e.g., some tricyclic antidepressants, antihistamines, steroids, ephedrine)
- Hives or other pervasive skin conditions (e.g., eczema)exist
- Direct exposure to a suspected allergen may cause anaphylactic shock
- A patient shows excessive anxiety about a skin test
The test may also be ordered if the patient is an infant or after a positive skin test to many foods.
An allergy can only be treated successfully when the cause of the allergy is known. The radioallergosorbent test (RAST) and related tests (e.g. RASP, FAST) are blood tests performed to determine the specific cause of an allergy.
The RAST looks for specific antibodies called immunoglobulin E (IgE), which are produced in response to an allergen. The body’s immune system produces a number of different antibodies to defend against intruders (called antigens).
An allergy occurs when the body overreacts to a substance that is normally harmless. A person who is allergic to pollen, for instance, produces IgE antibodies to “defend” against pollen molecules that enter the body.
These IgE antibodies bind to white blood cells called mast cells and basophils. When they come into contact with a pollen molecule, mast cells and basophils explode, releasing chemicals such as histamine. Histamine binds with receptors in the nose and other areas of the body, causing allergy symptoms (e.g., sneezing, runny nose, itching).
The best way to treat an allergy is through avoidance, which is possible only when the allergen is identified. The RAST uses radioactive or enzyme markers to detect levels of IgE antibodies in the blood. Every specific allergen is selected and tested separately each time. The test measures levels of particular allergens (e.g., egg white versus egg yolk, giant ragweed versus western ragweed).
These tests measure the amount of the suspected IgE antibody in the blood that reacts to each allergen and reports it as a numerical value. The added radioactive substance essentially binds itself to the allergen-antibody in the patient’s blood and highlights the amount of antibody that is responding to the introduced allergen.
When the RAST test is completed and all the suspected allergens have been tested, the allergen or substance that is causing an allergic reaction should be determined. However, there are circumstances when a retest with other allergens or other tests need to be performed to discover the offending allergen.
The RAST test and other blood tests are generally comparable in accuracy to skin prick tests, with both types of tests producing results that are about 90 percent accurate. However, the RAST may not be as accurate when testing for food allergens because the level of antibody in the blood does not necessarily indicate the severity of the allergic reaction in the specific individual. Even low level test results can be associated with a severe reaction. Instead, the RAST test is used to predict the likelihood of a reaction. The lower the level, the less likely a reaction is to occur.
Although both types of tests are considered precise, skin tests are more commonly used because they are less expensive and can produce results within 20 minutes. Blood test results take longer because blood must be sent out to a laboratory for analysis.
RAST is used instead of a skin test when:
- Certain medications can suppress or interfere with skin test results (e.g., some tricyclic antidepressants, antihistamines [including nasal antihistamines and over-the-counter antihistamines], steroids, ephedrine)
- Hives or other pervasive skin conditions (e.g., eczema) exist and may obscure results
- Direct exposure to a suspected allergen may cause anaphylactic shock
- Patient anxiety about skin tests persists
A RAST test may also be ordered if the patient is an infant or after a skin test suggests an allergy to many foods.
Avoiding the allergy causing substance and determining if allergy symptoms cease or persist will ultimately determine if an allergy test was accurate.
Types and differences of allergy blood tests
Many physicians refer to allergy-related blood tests that test immunoglobulin E (IgE) levels as a RAST but this may not be the exact testing method that the lab is using. There are other blood tests that may be used. For example, some tests will measure other substances in the blood, which may be more effective at pinpointing particular allergens.
Other types of blood tests include:
- CAP-RAST FEIA.This blood test is similar to RAST but is more useful in determining reactivity to certain foods.
- Radio allergo sorbent procedure (RASP).This blood test is a variant of the RAST but is more sensitive to food allergens. RASP may measure some immunoglobulin G (IgG) complexes in addition to IgE. However, the role of IgG complexes is not entirely clear and this test may not always provide valid results.
- Fluorescent allergo sorbent test (FAST). This blood test is similar to RAST but uses fluorescent instead of radioactive compounds to test the blood. This makes the equipment well-suited for use in an office (instead of a laboratory), where results can be delivered in about six hours.
- Multiple antigen simultaneous test (MAST). A spinoff of the RAST that proponents believe is more comprehensive and sensitive than its predecessor. MAST testing allows physicians to test for 38 allergens at a time (although it cannot diagnose food allergies), whereas RASTs look at only a single allergen per test.
- Immunoassay capture test.One of the newest blood tests. Proponents say the process used to make the patient’s blood and the test medium react leads to a result as sensitive and accurate as those obtained from allergy skin testing.
- Paper radioimmunosorbent test (PRIST). This test measures the total number of IgE antibodies in the blood.
- Complement testing. Complement is made up of proteins that bind with antibodies, and is involved in immune system functioning. Patients with hives or angioedema often show a deficiency in blood complement. Complement levels also can help a physician determine whether a case of angioedema is related to an allergy or to hereditary factors. This is important, because treatments differ for the two conditions.
Though the RAST and related blood tests can help diagnose an allergy, not all blood tests are reliable. Complementary and alternative medicine allergy tests (e.g., cytotoxic testing), for instance, do not provide reliable or useful diagnostic results.
Before the RAST test
Patients should inform their healthcare provider of any prescribed or over-the-counter medication they are taking prior to the test, as well as any other issues (such as diabetes). The physician will discuss with the patient any preparations that are needed prior to the test.
On the day of the test, the procedure will be explained and patients will have the opportunity to ask questions. The medical staff will also ask questions about the patient’s medical history and lifestyle to determine if the patient is taking any medications or has other issues that will interfere with the test’s accuracy.
Personal medical histories contain the most important information about a patient’s health history, including:
- Diseases and disorders
- Medications, tests and vaccinations
- Lifestyle issues related to health
- Past hospitalizations and surgeries
- Symptoms that may indicate undiagnosed disorders
- Family history
Detailed documentation can contribute to achieving accurate and useful test results. Patients should especially try to include:
- Specific foods consumed.
- Places visited often (e.g., garden, farm, woods, construction site, basement).
- Products used regularly (e.g., soaps, colognes, perfumes, deodorants, cosmetics, detergents, pesticides, household cleaners).
- Illnesses, health disorders and surgeries and the dates they occurred. Exact diagnoses should also be noted.
- Job occupation and place of residence.
It may be helpful for a patient to prepare an allergy diary. An allergy diary is a written record of all possible allergens to which a patient has been exposed and the approximate time of exposure. Any symptoms that occur after exposure are recorded, no matter how minor. The nature of symptoms – including severity, duration and evolution – are also recorded. It is important that this information be recorded at the time of allergen exposure, because retrospective diaries often miss valuable information.
Prior to the test, the physician and patient may selectively choose individual allergens suspected of causing allergies from a wide-ranging list. Groupings of tests can be conducted that focus on specific allergens. For example, nuts and corn or plants such as regional grass and weeds.
During the RAST test
During the test, a blood sample is drawn. If the physician needs only a drop or two of blood, then a simple prick of the finger, earlobe or heel (in the case of an infant) can provide enough blood for testing. However, most allergy and asthma-related blood tests require a larger blood sample. For most blood tests, the sample is drawn from a vein in a process called venipuncture. Blood samples may be more difficult to obtain from infants, overweight people whose veins are difficult to find, elderly people whose veins tend to roll away or patients with scarred or collapsed veins due to multiple transfusions or intravenous drug use.
During a venipuncture, a health provider wraps an elastic band around the patient’s upper arm to stop blood flow through the upper veins. This keeps the lower veins full of blood and less likely to collapse, making them ideal sites for drawing a sample.
Once a precise site is chosen, the spot is swabbed with alcohol. The needle is inserted into the vein. In some cases the needle may have to be removed and inserted again to ensure it is properly placed, or if the health provider cannot obtain enough blood from the original site. Patients may feel a brief sting as the needle is inserted, but discomfort is usually minor.
Once the needle is in place, a collection tube is attached and blood flows into it. Sometimes, more than one tube will be collected. Once the required amount of blood has been obtained, the rubber band is removed. The needle also is removed from the vein and a cotton ball or gauze pad is applied to the puncture site. Direct pressure is applied to the puncture spot for several minutes to help the blood clot, and a sterile bandage is placed over the site.
The blood sample is placed in tubes, vials or is placed on specially treated paper and sent to a lab. There, the sample will be analyzed to measure the amount of immunoglobulin E (IgE) in the blood and to identify specific allergens.
There are very few risks or side effects reported with blood tests. Occasionally, bruising is reported at the injection site. This can be minimized by keeping direct pressure on the spot for several minutes after the needle has been removed.
In rare situations, the vein used to obtain the sample may become inflamed (phlebitis). Applying warm compresses to the spot several times daily can help reduce the swelling.
Patients with bleeding disorders or who are taking anticoagulants (including aspirin) may have problems with continued bleeding. Patients who experience these side effects should consult with their physician.
After the RAST test
Following the withdrawal of blood, patients may resume medications and food intake according to their physician’s orders. Immediately after the blood sample is taken, the test tubes or paper are labeled with the date and the patient’s name, and they are sent to the laboratory for testing.
Results can be returned within days or several weeks, depending on which tests need to be performed. Based on the test results, additional blood tests may be ordered.
The meaning of the RAST results
Test results are always evaluated in relation to the “normal range” for that test. The range of values considered to be normal is the range of test results from the blood of normal, active healthy people.
When someone has a disease or health problem, the blood test results may be higher or lower than normal (“outside of the normal range”). When a physician sees that a blood test is outside of the normal range, a repeat test may be ordered to verify the results or additional tests may be required to determine the underlying causes behind the abnormality.
It is important to realize that RAST (or any other blood test) results are not absolute. Though positive results most probably indicate that the patient has a true allergy (an IgE response to that specific allergen), the results must always be interpreted by the specialist or physician. Results ultimately depend upon the individual’s response in conjunction with specific symptoms and medical history. IgE levels may indicate an allergic reaction is taking place but if there are no physical symptoms being experienced, the individual does not have an allergy. If an individual’s IgE test is negative, there is still a small possibility that the individual does have an allergy. In addition, the level of IgE present does not predict the potential severity of an allergic reaction in the patient.
Once an allergen is identified the offending substance should be avoided by the patient. Test results can easily be cross–checked by experience – if the offending substance is avoided and the patient gets healthier the procedure worked; if the patient does not get better it did not. A retest or another test or method may be needed to determine which allergen(s) are causing the symptoms to persist.
When an allergy test pinpoints which allergen is causing an allergic reaction an allergist/immunologist will use the information to develop a treatment plan. The patient may be advised to make certain lifestyle modifications that may be effective in preventing symptoms on a day-to-day basis. Over-the-counter and/or prescription medications (e.g., antihistamines, decongestants) may also be recommended. Finally, if all other methods fail to treat the problem, allergy shots (immunotherapy) may be recommended.
Questions for your doctor about the RAST test
Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to the RAST test:
- Why are you recommending that I undergo a RAST test?
- Do I need to do anything to prepare for the test?
- How long will my test take?
- Do I face any risks by undergoing a RAST test?
- Am I likely to experience pain during the test?
- When will my results be available?
- Will you call me when the results are in?
- What do my results indicate?
- How accurate is this test? Can I trust the results?
- Now that my results are in, what is the next step?
- Do I require further testing?