The Radioallergosorbent test, or RAST, is a blood test designed to identify substances to which a person may be allergic by detecting specific antibodies in the bloodstream. Developed in the 1960s, this laboratory test was one of the first methods to provide an alternative to traditional skin-prick testing for diagnosing allergies. Its development was a significant step in understanding how the immune system responds to allergens.
The Purpose and Process of the RAST
The purpose of the RAST is to measure the concentration of specific Immunoglobulin E (IgE) antibodies in a blood sample. The immune system produces these IgE antibodies as part of an allergic response to a substance it mistakenly identifies as a threat, such as pollen, pet dander, or certain foods. By quantifying the amount of IgE directed against a particular allergen, the test helps to pinpoint potential triggers for a person’s allergy symptoms.
The testing process begins with a blood draw. In the laboratory, the liquid portion of the blood, called serum, is isolated. This serum is then exposed to an insoluble material to which a suspected allergen, like peanut protein or house dust mite particles, has been bound.
If the patient’s serum contains IgE antibodies specific to that allergen, they will attach to the allergen-coated material. To measure the amount of bound IgE, a second antibody is introduced. This antibody is specifically designed to bind to human IgE and has been labeled with a radioactive isotope.
After this radiolabeled antibody attaches to the IgE already bound to the allergen, any unbound antibodies are washed away. The final step involves measuring the level of radioactivity, which is directly proportional to the quantity of allergen-specific IgE present in the original blood sample.
Interpreting RAST Results
RAST results are reported using a class system, often on a scale from 0 to 6. A result of Class 0 is considered negative for the presence of specific IgE antibodies. Higher class numbers indicate progressively greater concentrations of those antibodies. For instance, a Class 1 might show a low level of IgE, while a Class 5 suggests a very high level.
It is important to understand that a positive RAST result signifies sensitization to an allergen, which is not the same as having a clinical allergy. Sensitization means that the body has produced IgE antibodies in response to a substance, but it does not guarantee that exposure will lead to physical symptoms. A person can be sensitized without ever experiencing an allergic reaction.
The level of IgE detected by the test does not consistently correlate with the potential severity of an allergic reaction. A high IgE level does not automatically mean a person will have a severe reaction, and a lower level does not guarantee a mild one. For this reason, a physician must interpret the results alongside the patient’s clinical history and reported symptoms to make an accurate diagnosis.
Comparison with Skin Prick Testing
The RAST is often compared to the skin prick test when evaluating allergies. The procedures are different; the RAST requires a blood draw, while a skin prick test involves applying tiny amounts of allergen extracts to the skin and then lightly pricking the surface. This difference makes blood testing a suitable option for individuals who cannot undergo skin testing.
Patients with extensive skin conditions like severe eczema or those who cannot safely discontinue antihistamine medications, which would interfere with skin test results, may be candidates for a blood test. The RAST also carries no risk of inducing an allergic reaction, as it is an in-vitro test performed on a blood sample. In contrast, skin testing, while generally safe, carries a very small risk of causing a systemic allergic reaction.
There are also practical differences in speed and cost. Skin prick test results are available within 20 minutes in a doctor’s office. RAST results must be sent to a laboratory for analysis, a process that can take several days. The RAST is also more expensive than skin prick testing.
Modern Alternatives and the Evolution of Allergy Testing
The original RAST that used radioactive isotopes is now considered outdated technology. It has been succeeded by more advanced blood tests. The most common modern methods are types of enzyme-linked immunosorbent assays (ELISA), with the fluoroenzyme immunoassay (FEIA) being a prominent example, often recognized by its brand name, ImmunoCAP.
These modern assays are preferred for several reasons. An advantage is that they do not use radioactivity, which simplifies the laboratory process and eliminates concerns associated with handling radioactive materials. Tests like the ImmunoCAP offer greater sensitivity and specificity compared to the original RAST. This means they are better at correctly identifying the presence of specific IgE (sensitivity) and less likely to produce false-positive results (specificity).
The quantitative results from modern tests are also more precise, providing a specific numerical value of IgE concentration rather than a general class or range. While the term “RAST test” is still used colloquially by patients and some healthcare providers, the actual test being performed today is one of these superior, non-radioactive alternatives. This evolution reflects a trend in medical diagnostics toward safer and more accurate methods.