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RAST Test – Allergy Diagnosis via Blood Test
The RAST test is a blood test used to diagnose allergies by measuring specific IgE antibodies against particular allergens in the bloodstream.
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The RAST test is a blood test used to diagnose allergies by measuring specific IgE antibodies against particular allergens in the bloodstream.
What is the RAST Test?
The RAST test (Radioallergosorbent Test) is a laboratory diagnostic procedure used to identify allergies. It measures whether -- and to what extent -- a patient's immune system has produced specific IgE antibodies against certain allergens. IgE antibodies are proteins of the immune system that play a central role in allergic reactions. Originally developed in the 1970s using radioactive markers, the RAST test has largely been replaced by modern, non-radioactive methods such as the ImmunoCAP test (also ELISA-based assays), which operate on the same principle and are often still referred to collectively as the RAST test.
How Does the RAST Test Work?
A small blood sample is taken from the patient. In the laboratory, the blood sample is exposed to various allergen extracts. If the patient is sensitized to a specific allergen, the specific IgE antibodies present in the blood will bind to it. The amount of bound IgE antibodies is then measured and reported in classes or concentration units (kU/L).
Result Classes
- Class 0: No detectable sensitization
- Class 1: Borderline sensitization
- Class 2: Mild sensitization
- Class 3: Moderate sensitization
- Class 4-6: Strong to very strong sensitization
Important: A sensitization (positive test result) does not necessarily mean that clinical allergy symptoms are present. Results must always be interpreted in the context of the patient's medical history and complaints.
When is the RAST Test Used?
The RAST test is primarily used when a Type I allergy (IgE-mediated allergy) is suspected. Typical indications include:
- Suspected pollen allergy (hay fever)
- Suspected food allergy (e.g., nuts, milk, egg, wheat)
- Suspected pet dander allergy
- Suspected house dust mite allergy
- Suspected insect venom allergy (e.g., bee or wasp venom)
- Suspected mold allergy
- Evaluation in cases of severe allergic asthma
The RAST test is particularly suitable when a skin test (prick test) cannot be performed -- for example, in patients with extensive skin conditions, those taking antihistamines, or those at elevated risk of anaphylactic reactions.
Advantages Over Skin Testing
- No need to discontinue medications (e.g., antihistamines)
- Suitable for patients with skin conditions such as eczema or psoriasis
- No risk of an allergic reaction during testing
- Objective, quantitative measurement of IgE concentration
Limitations and Disadvantages of the RAST Test
- Less sensitive than the prick test for certain allergens
- No immediate visible reaction -- results are only available after several days from the laboratory
- Higher cost compared to skin testing
- A positive result does not automatically confirm a clinically relevant allergy
Diagnosis and Clinical Interpretation
The RAST test is typically ordered and evaluated by an allergist or immunologist. Results are always assessed in the context of the patient's medical history, symptoms, and, if necessary, additional diagnostic procedures such as the prick test or a controlled provocation test. Interpreting RAST test results in isolation, without clinical context, is not sufficient for a reliable allergy diagnosis.
References
- Matricardi PM et al. - EAACI Molecular Allergology User's Guide. Pediatric Allergy and Immunology, 2016.
- Muraro A et al. - EAACI food allergy and anaphylaxis guidelines: diagnosis and management of food allergy. Allergy, 2014.
- Allergy and Asthma Network - Understanding Allergy Testing: RAST and ImmunoCAP. allergyasthmanetwork.org, 2022.
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