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Allergen Specific IgE

Category Immunology
Test background

Specific IgE tests (previously known as a RAST): These tests are for the investigation of IgE mediated hypersensitivity (allergic) reactions. Specific IgE testing should be targeted only at triggers (aeroallergens/venom/food/drugs) of immediate hypersensitivity (urticaria, angioedema, pruritus, wheeze) identified by a thorough clinical history. The laboratory does not recommend the testing of more than 10 allergens per patient.

A positive specific IgE result does not confirm allergy, only sensitisation to the allergen in question. These tests can help to confirm a diagnosis of an allergic disorder in patients with a clinical history consistent with an immediate allergic reaction. They are particularly useful when skin testing cannot or should not be performed (dermatographism, extensive eczema, patients unable to stop anti-histamines or tricyclic anti-depressant medication). All patients with anaphylaxis should be referred to an allergy clinic according to NICE guidance.

False positive specific IgE results can occur in patients with high total IgE levels (IgE >1000 KUA/L). Low level positive specific IgE results in these patients should be interpreted with caution. A negative specific IgE does not exclude IgE mediated allergy where there is a convincing clinical history.

Aeroallergen IgE Testing: Common aeroallergens in the UK include grass, tree, weed pollens, house dust mite, cat and dog dander and fungal spores. IgE testing is recommended for the diagnosis of allergic rhinitis which affects up to 20% of UK adults. Pollen sensitisation can lead to the pollen food allergy syndrome which can often be diagnosed on clinical history alone.

Food Allergy IgE Testing: Indiscriminate specific IgE testing is strongly discouraged due to the limited specificity of food IgE tests and their potential to promote anxiety, the unnecessary elimination of food leading to nutritional and growth disorders and reduced quality of life. Unnecessary elimination diets can induce allergic disease including anaphylaxis to previously tolerated foods. Symptoms (urticaria, angioedema, eczema flares) that have no close temporal association to food (onset at night or early morning more than 2 hours after food has been consumed) are unlikely to be IgE mediated. Atypical presentations and reactions following composite meals should be referred to an Allergy clinic. In all cases only food reported to provoke symptoms should be tested. The laboratory recommends testing a maximum of 10 food allergens per patients.

Drug IgE Testing: The laboratory does not recommend penicillin IgE testing within primary care. This test has limited sensitivity and specificity for penicillin allergy and clinicians are advised to first use the PEN-FAST tool for assessment of penicillin allergy risk.

Component resolved IgE testing: The laboratory also offers component resolved diagnostics (CRD). These tests detect IgE to specific components of an allergen. CRD is useful in delineating pollen food syndrome from primary food sensitisation, for the confirmation of LTP sensitisation and for the selection and monitoring of oral immunotherapy.

Specific IgE testing is performed by fluoroenzymeimmunoassay (Immunocap) on Phadia.

Clinical Indications

Investigation of type 1 hypersensitivity (allergic) reactions where skin testing is contraindicated.
Selection of patients for immunotherapy.

Reference range

<0.35 kUA/L (Measurement range 0.1 – 100 kAU/L)

This range originates from the detection limit of the first assays available, new assays now have lower detection limits and some patients will react to an allergen when specific IgE is low (0.10—0.34kUA/L). A positive result does not indicate allergy, only the potential to react to an allergen and the magnitude of a sIgE result does not correlate with clinical severity, only to the likelihood of clinical reactivity to the allergen in question.

Defined cut offs where the probability of clinical allergy is very high have been described for some foods, e.g. peanut >15 kUA/L (BSACI 2017).

 

 

Sample & container required Serum (rust top RST tube or yellow top tube)
Sample volume 5-10 mL blood (1 mL serum)
Turnaround time 14 Days
Notes

Rare allergens not tested by the laboratory are referred to Biomnis.

Currently awaiting accreditation for component testing.

 

Information and advice for GPs about the use of IgE blood tests and changes to the IgE testing process and interpretative reports:

Allergy Testing Update

Advice on diagnosis and assessment of food allergic disorders

Food Allergy Syndromes

Penicillin Allergy