Allergy / hypersensitivity interpretation:

For all specific IgE (sIgE, formally called RAST) test requests the allergen(s) selected must be based on the particular clinical history of the patient.  A detailed allergy-focused history should be able to identify likely causes of specific symptoms.  Blanket testing of allergens not based on history are of no clinical utility.

Allergy cannot be diagnosed on the basis of sIgE tests alone.

A positive sIgE confirms sensitisation to the allergen tested.  This sensitisation may or may NOT be responsible for the cause of symptoms.  Sensitisation can occur without the presence of clinical symptoms and may or may not have a bearing on the likelihood to developing allergic disease.

A negative sIgE does NOT exclude allergy.

A high total IgE concentration (>1000 KUa/L) can result in false positive specific IgE results.  This is often found in patients with atopic dermatitis resulting in weakly positive results to multiple allergens due to non-specific IgE binding.

Very low total IgE can cause false negative results.

Measurement of sIgE in blood may be preferable to skin prick testing (SPT) in certain clinical situations; for example in patients with widespread skin disease (e.g. eczema), patients receiving skin test suppressive therapy (including H1-antagonists and tricyclic antidepressants), uncooperative patients or those with a greater risk of anaphylaxis.  They may also be used where there is an absence of healthcare professionals who can perform and interpret SPTs.

Eczema

Use of sIgE in patients with atopic dermatitis may be preferable to SPT.  However please be aware that these patients commonly exhibit high total IgE concentration (>1000 KUa/L) therefore the utility of positive sIgE is doubtful as they may be false positive and will need careful interpretation. sIgE to house dust mite is often high and house dust mite allergy may exacerbate eczema in such patients. 

Rhinitis

Serum sIgE should be requested when skin tests are not possible or when SPT, together with clinical history give equivocal results.  The range of allergens tests should be guided by a careful history but generally includes allergens to which people are commonly exposed. These include house dust mite (HDM) grass pollen and cat dander.  There is little point in identifying the exact pollen unless desensitisation (immunotherapy) is intended.

If rhinitis or asthma symptoms are seasonal the following pollens may be causative:

Tree (Silver birch) March – May

Grasses (May – Sept)

Weeds (July – September)

Asthma

House dust mite sIgE is commonly associated with extrinsic asthma.  In addition sIgE to animal dander (cat, dog or horse etc) may be appropriate.

About 15-20% of allergic asthmatics suffer from Aspergillus-induced allergies.  IgE to Aspergillus is associated with the need for closer monitoring and can be linked to other conditions such as allergic bronchopulmonary aspergillosis (ABPA).

Food

Food allergy can be classified into IgE-mediated and non-IgE mediated. IgE testing will only be helpgul in an IgE mediated reaction is suspected. Key parts of the history are the Symptoms, Timing, whether the symptoms can be attributed to a likely Allergen and their Reproducibility (STAR). 

Lab-Testing-Guideline-13-Jan-2025.pdf

Venoms

Hymenoptera insects that are commonly responsible for human stings include honeybees (Apidae family) and wasps (Vespidae Family).

Specific IgE testing is not indicated for local reactions. 

Patients with systemic hymenoptera reactions should be referred to a Clinical Allergy service. Specific IgE testing and baseline tryptase level can be checked at the time of  referral. 

Component Resolved Allergy Diagnostics  

Interpretation of sIgE results can be confounded due to the presence of cross-reacting antibodies.

Recently the individual components of native allergens have been produced due to advances in molecular biology.  The characterisation and production of recombinant forms of these allergens at the molecular level has enabled their use in diagnosis.

Cross-reactions may be due to antibodies recognising similar epitopes from proteins of different species e.g. the major birch pollen protein Bet v 1 (after the Latin for birch Betula verrucosa) and its homologues (e.g. Mal d 1 from Malus x domesticus [apple] which gives rise to the so-called ‘oral allergy syndrome’or ‘pollen food syndrome’ (PFS). Reactions due to PFS tend to be mild, without systemic features, and respond promptly to antihistamine. 

Table 1: listing some key allergen component associated with highly significant allergy (not exclusive)

Should you ask for component tests?

 Interpretation needs specialist training and experience with maintenance of CPD in allergy (e.g. Attending annual meeting of BSACI/EAACI/AAAI). These tests should usually be requested from specialist allergy clinics only. 

References

  1.      National Institute for Health and Clinical Excellence (2011) Food allergy in under 19s: assessment and diagnosis. Clinical Guideline 116. London: NICE

  2.      Nicolaou N, Poorafshar M, Murray C et al Allergy or tolerance in children sensitized to peanut: Prevalence and differentiation using component-resolved diagnostics J Allergy Clin Immunol 2010;125:191-7

3.      Sampson HA. Utility of food-specific IgE concentrations in predicting symptomatic food allergy.   J Allergy Clin Immunol 2001;107:891-6.

 4.      Fleischer DM, Conover-Walker MK, Christie L, Burks AW, Wood RA. The natural progression of peanut allergy: resolution and the possibility of recurrence. J Allergy Clin Immunol 2003;112:183-9

Useful Links

Allergy UK: www.allergyuk.org

Asthma UK and British Lung Foundation : https://www.asthmaandlung.org.uk/

National Eczema Society:  Home - National Eczema Society

Anaphylaxis Campaign:  www.anaphylaxis.org.uk

Guidelines for the use of laboratory allergy testing in primary care: www.bsaci.org