IgE-mediated allergy involves the rapid onset of symptoms following exposure to an allergen, which is usually a naturally occurring protein. The symptoms are mediated by histamine and various other mediators, which are released by activated mast cells following the ligation of surface-bound IgE receptors by allergen.
Airborne allergens trigger symptoms in the eyes (conjunctivitis), nose (rhinitis) and small airways (wheeze). Foods, medications, insect venoms and latex may trigger ‘systemic’ allergic reactions with features such as pruritis, urticaria, angioedema, airway compromise and low blood pressure – in its severe form known as ‘anaphylaxis’. Food allergens (particularly fresh fruits, nuts and vegetables) may trigger a local allergic reaction in the mouth and oro-pharynx known as ‘oral allergy’, which is usually experienced as oral itching.
Sensitisation to allergens may be investigated in the clinic using skin prick testing, but an easily accessible alternative is the detection of allergen-specific IgE in blood. The performance of allergen-specific IgE for airborne allergens is very similar to skin prick testing. For certain other allergens there can be differences – please contact the laboratory in case of questions.
Allergy tests are relatively straightforward to request and interpret for simple situations such as animal dander allergy and seasonal rhinitis. It becomes much more complex in chronic atopic diseases such as chronic rhinitis, chronic asthma and eczema. Patients with these conditions frequently have positive tests, but the contribution of the sensitisation to their clinical picture is complex and requires interpretation. ‘False-positive’ allergy tests where people don’t have an associated disease are very common, particularly with serological tests. It is seen particularly in the setting of birch pollen allergy: the major allergen is a ‘panallergen’ found in fruits, nuts and vegetables, therefore birch-sensitised individuals (at least 10% of adults) will often return positive results for multiple food items due to laboratory cross-reactivity. Some patients may experience ‘oral allergy’ on exposure, however the allergy test results show poor correlation to the foods involves. Many others are non-reactive. Allergy test requesting and interpretation must therefore be very targeted and clinically-based.
One way of improving diagnostic performance is by using ‘molecular allergy serology’. A wide variety of such tests are now available through the laboratory, generally reserved for specialist use.
The ALEX explorer chip is used to investigate sensitisation to most commercially available allergens using a combination of native allergens and allergen molecules; a gold top sample is required with turnaround of around 4 weeks. This test is available to the Allergy Clinic; other users may access following discussion with the Allergy team.
False-negatives may also be observed. Where clinical suspicion is high please refer or send an advice and guidance request to the allergy clinic.
Some common requesting scenarios are listed below:
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Seasonal rhinitis or seasonal wheeze |
Test for IgE to the relevant seasonal allergens (spring - tree pollen; summer - grass pollen; late summer/ autumn - alternaria mould spores). Investigation is only relevant in a specialist setting where desensitisation is required - otherwise the results don't change management. |
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Oral allergy syndrome |
A common food allergy caused by specific IgE to tree pollens (in particular Silver Birch) that cross-react with fruits, vegetables and nuts. Cooked fruits are tolerated. Tests for fruits are not useful for routine management, as false negatives/ positives are common and results do not correlate with the foods causing symptoms. A clinical diagnosis based on typical symptoms in patients with hayfever is sufficient. Where nuts are involved or reactions are more systemic in nature, please refer to the allergy clinic. |
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Chronic (perennial) rhinitis or chronic asthma |
Allergy testing in the setting of chronic (year round, perennial) rhinitis has been shown to change management and improve outcomes. Tests for dust mite, mould mix and any household pets are recommended to guide advice on allergen avoidance (see Allergy UK website) and potentially referral for desensitisation. Allergen avoidance is less useful for asthma without chronic rhinitis. For specialist use, the tests can be used to phenotype asthma patients as 'atopic' or 'non-atopic' which guides the use of biologics. Aspergillus IgE may also be used along with total IgE and aspergillus IgG for investigation of possible APBA. |
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Eczema |
Although eczema is an atopic disease, allergy approaches (testing, avoidance, desensitisation) have not shown any benefit for the skin disorder itself and are not routinely recommended. Tests may be needed for co-morbidities such as rhinitis and genuine IgE-mediated food allergy. |
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Chronic spontaneous urticaria and / or angioedema |
These conditions are rarely due to specific IgE mediated disease. Test for individual allergens, if relevant, as determined by the patient history / symptom diary. Note that ACE inhibitors may induce intermittent angioedema despite daily drug ingestion. The mechanism here is not IgE-mediated. |
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Anaphylaxis and milder forms of systemic allergic reaction |
Explore triggered symptoms vs. spontaneous urticaria by clinical history; true allergic reactions have rapid onset and are discrete/ dramatic events, usually with more than just skin features. Spontaneous urticaria tends to be more of an ongoing skin issue, although some patients may experience intermittent exacerbation including angioedema. Where the problem does appear to be triggered, consider foods, medications and insect venoms rather than airborne allergens. Test for individual allergens where relevant. For nut allergy a combined nut panel may be used with individual nuts tested where the combined nut panel is positive. A similar panel may be used for fish or shellfish. Some patients have anaphylaxis only if food ingestion is followed by exercise. This is a specialist field and possible cases should be referred to the allergy clinic. |
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Drug allergy |
There are limited tests available for drugs and antibiotics. It is important to note that the sensitivity of specific IgE for these allergens is low i.e. a negative test does not exclude allergy. |
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Natural rubber latex allergy |
Specific IgE for immediate hypersensivity (not irritant or contact allergic dermatitis) may aid in the diagnosis but this assay is not 100% sensitive for latex allergy. Thus a negative test does not exclude allergy. Latex allergy may be associated with allergy to tropical fruits (especially banana, kiwi, avocado). |
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Bee and wasp venom allergy |
Many individuals stung by these insects will develop IgE antibodies to the venom but the risk of anaphylaxis with future stings is not defined by these tests. These should only be tested to confirm the presence of specific IgE to the venoms in support of commencement of specific immunotherapy (desensitization) for anaphylaxis. |
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Samples required |
● Clotted blood (gold cap, 5 mL tube). |
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Interpretation |
< 0.35 kUA/L is considered negative. ≥ 0.35 kUA/L is reported as positive. Detectable specific IgE below 0.35 kUA/L may be clinically relevant in very young children in the right clinical context. |
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Turnaround time |
14 days |
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Enquiries |
Immunology |