Specific IgE Blood Test

Specific IgE Blood Test

Specific IgE tests (formerly called RAST) are used to examine IgE-mediated hypersensitivity (or allergic) responses. These tests need to be targeted at potential triggers (such as aeroallergens, food, venom, or drugs) that cause instant hypersensitivity reactions like urticaria, pruritus, angioedema, or wheezing based on a comprehensive clinical history. The laboratory recommends testing no more than 10 allergens per person.

A positive result indicates sensitization to the allergen, but it is not a confirmed allergy. These tests are helpful in diagnosing allergic disorders in those with a clinical history suggesting an immediate allergic response. They are predominantly valuable when skin testing is not feasible or advisable, such as in cases of extensive eczema, dermatographism or for patients who cannot stop taking antihistamines or tricyclic antidepressants.

NICE guidance shows all persons with anaphylaxis must be referred to an allergy clinic.

False-positive results may happen in those with elevated total levels of IgE (IgE >1000 KUA/L), and low-level positive results in these cases require careful interpretation. A negative specific IgE test does not rule out IgE-mediated allergy if there is a strong clinical history.

Aeroallergen IgE Testing:

Prevalent aeroallergens are grass, weed pollens, tree, cat and dog dander, house dust mites, and fungal spores. The igE test is suitable for diagnosing allergic rhinitis affecting up to 20 percent of adults in the UK. Pollen sensitization is likely to result in pollen food allergy syndrome, often diagnosable based solely on clinical history.

Food Allergy IgE Testing:

Indiscriminate IgE testing for food allergies is not recommended because of the restricted specificity of food IgE tests. This can lead to unnecessary anxiety, elimination of foods that may cause nutritional or development issues, and a poor quality of life. Avoiding foods unnecessarily may also trigger allergic reactions to formerly tolerated foods, like anaphylaxis.

Symptoms such as angioedema, urticaria, and eczema flares that are not closely linked to food intake (such as those occurring at night or more than 2 hours after eating in the morning) are not likely to be IgE-mediated. Atypical reactions or symptoms from composite foods necessitate referral to an allergy clinic. Only foods that have been reported to trigger symptoms need to be tested, with a recommended maximum of ten food allergens per individual.

Drug IgE Testing:

Penicillin IgE testing is not advisable in primary care due to its restricted sensitivity and specificity. Clinicians often consider first using the PEN-FAST device to assess the risk of penicillin allergy.

Component-Resolved IgE Testing:

The lab provides component-resolved diagnostics (CRD), which identify IgE to precise allergen components. CRD is vital for distinguishing between pollen food syndrome and primary food sensitization, confirming LTP sensitization, and choosing and monitoring oral immunotherapy.

Testing Method

Specific IgE testing is conducted using fluoroenzyme immunoassay (ImmunoCAP) on Phadia.

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