Allergy Investigations
Allergy is a clinical diagnosis. Specific IgE (Previously known as RAST) testing can help where there is a clear history of immediate/type I hypersensitivity reactions to a specific allergen. Specific IgE testing is not essential, this should only be considered if it would change clinical management.
There is no role for ‘screening’ with multiple specific IgE tests.
Specific IgE tests have no role in assessing for Eczema triggers. Where allergy is suspected to be the cause for eczema, dermatology would consider patch tests for delayed/Type 4 hypersensitivity. Patch tests are available for many contact allergens such as cosmetics. Patch tests are not available for food. Eczema in general is not due to food allergy but rarely food hypersensitivity can contribute to eczema in infancy. If there is a strong suspicion that a specific food item is contributing to eczema, management would be trial avoidance of the suspected food trigger followed by re-introduction few weeks later to establish association: ideally under the guidance of dietetics. Specific IgE testing would be inappropriate and can give false positive results leading to unnecessary food avoidance.
Many reactions to foods and chemicals are not IgE-mediated. There are no blood tests available for this type of reaction. An example would be lactose intolerance due to lactase deficiency. Tolerance of lactose free milk essentially excludes cow’s milk protein allergy.
Possible allergens include foods, inhalants, drugs and bee/wasp venom.
Key points
- Prior tolerance does not exclude allergy
- Symptoms occur within minutes to an hour of exposure to a trigger
- Symptoms are reproducible and occur with every subsequent exposure
- Symptoms do not occur without exposure to the allergen
Clinical features
- Rash (urticaria or erythema) typically within 1hr from allergen exposure. Persistent rash lasting for days would not be from IgE mediated allergy. If frequent episodes with no consistent trigger, consider spontaneous/idiopathic urticaria. Consider physical causes for urticaria such as heat, sweating, cold, pressure, trauma/dermographism.
- Angioedema typically within 1hr from allergen exposure. Important to consider drug induced angioedema (ACE-I & NSAID). If isolated angioedema with no urticaria and it is not drug induced, consider Hereditary angioedema and refer to Immunology.
- Cough, wheeze, shortness of breath (Primary consideration would be asthma and other respiratory disorders. Allergic triggers for asthma such as pet dander and house dust mite should be considered. If these symptoms are accompanied by other symptoms such as oral itching, urticarial rash and angioedema food allergy should be considered)
- Rhinitis
- Vomiting and diarrhoea (Isolated GI symptoms are usually not from allergy but if these symptoms are accompanied by other symptoms such as oral itching, angioedema or rash, allergy as a cause should be considered)
Clinical history STAR
- Are the Symptoms consistent with an IgE mediated mechanism?
- Is the Timing consistent with an IgE mediated mechanism?
- Can the symptoms be attributed to a likely Allergen?
- Are the symptoms Reproducible?
Types of Allergy Tests
- Skin Prick Test: Measures immediate hypersensitivity to a specific allergen.
- Blood Testing (IgE Levels): Assesses the presence and level of IgE antibodies against specific allergens.
- Patch Test: Used for contact dermatitis, testing prolonged exposure to potential allergens.
Interpreting Allergy Blood Test Results
- Blood tests for specific IgE can identify sensitisation to a given allergen. They cannot confirm allergy in the absence of a relevant clinical history.
- Positive results suggest a higher likelihood of an allergic reaction but do not confirm causation.
- Negative results are less definitive and do not necessarily exclude allergy, particularly for drugs.
Food allergy
Follow the STAR approach and limit testing to the relevant trigger food. Any foods which have been subsequently eaten without provoking a response can be excluded from testing. It should rarely be necessary to test for more than 5 individual specific IgEs
Pollen food syndrome
Patient may experience mild oral symptoms after eating many raw foods such as nuts, fruits and vegetables. Symptoms are due to sensitisation to birch pollen and cross-reactivity with similar proteins in foods. If the patient is known to have hayfever and symptoms are mild there is no need to request blood tests for specific IgE. Exceptions to this are when the patient has symptoms after eating nuts or has more severe symptoms.
Urticaria
Isolated randomly occurring urticaria with or without angioedema is common. Most cases occur spontaneously and are not caused by allergy. Testing for specific IgE is not helpful in the absence of a clear and consistent trigger.
Referral to secondary care – see link to separate page
References
BSACI guideline for the diagnosis and management of pollen food syndrome in the UK 2022
