From the Guidelines
RAST allergy testing is no longer the preferred method for measuring specific IgE antibodies, having been replaced by more sensitive fluorescence enzyme-labeled assays, and the term RAST should be abandoned. A sample for allergy testing, particularly for identifying IgE-mediated food-induced allergic reactions, typically involves a blood test that measures the amount of specific IgE antibodies in the blood 1. The blood sample is sent to a laboratory where it's tested against various allergens such as pollen, pet dander, foods, insect venom, and medications.
Key Points to Consider
- The test results are usually reported on a scale, and the presence of sIgE reflects allergic sensitization and not necessarily clinical allergy 1.
- The predictive values associated with clinical evidence of allergy for one test method cannot be applied to other test methods 1.
- Undetectable sIgE levels occasionally occur in patients with IgE-mediated food allergy, and further evaluation, such as a physician-supervised oral food challenge, is necessary before telling a patient that they are not allergic to a suspected food 1.
- Allergen-specific IgE can be determined by testing the patient’s serum with an in vitro test, and using an immunoassay, allergen-specific IgE in serum is detected by incubating the serum with the suspected allergen 1.
Clinical Application
The choice of testing modality, including skin prick testing, blood or serum testing, intradermal testing, intradermal dilutional testing, or combinations thereof, depends on various factors, including the patient's condition and the suspected allergen 1. It is essential to interpret the test results in the context of the patient's clinical symptoms and medical history. Results typically take a few days to a week to come back, and the test is relatively painless with minimal risks beyond those associated with standard blood draws 1.
From the Research
RAST Allergy Testing
- RAST (Radioallergosorbent test) is a method used to measure the levels of IgE antibodies in the blood, which are responsible for triggering allergic reactions 2, 3, 4, 5.
- The test involves the use of a radioactive marker to detect the presence of IgE antibodies that are specific to a particular allergen 2, 5.
- RAST has been used to diagnose allergies to various substances, including timothy grass, nuts, and indoor allergens such as dust mites, cockroaches, and pet dander 2, 3, 4.
Technical Considerations
- The performance of the RAST test can be affected by various technical factors, including the speed of rotation of tubes, the number of washes, and the length of rotation of tubes 5.
- The quantity of anti-IgE used in the second step of the test and the quantity of antigen used in the coupling procedure are also important variables that can affect the results of the test 5.
- The choice of radioiodine, whether the tubes are rotated or not, the type of tube used, and the diluent employed can also impact the test results 5.
Comparison with Other Testing Methods
- RAST has been compared to other testing methods, such as skin testing, and has been found to be a useful tool for diagnosing allergies, particularly in cases where skin testing is not possible or is inconclusive 3, 4.
- However, RAST may not always be as sensitive as skin testing, and the results of the two tests may not always correlate 3.
- RAST can be useful for confirming the diagnosis of nut hypersensitivity in children or in highly allergic patients in whom skin testing poses a risk of anaphylaxis 4.