Specific IgE Panels

Allergen specific IgE testing measures serum IgE directed against individual allergens or allergen mixtures, by fluorescence enzyme immunoassay (ImmunoCAP), chemiluminescence or multiplex microarray. Results are reported in kUA/L (classes 0 to 6). The test shows sensitisation, which means the immune system has produced IgE to an allergen, and it does not by itself prove clinical allergy.

Interpretation always needs the history. Panels are grouped as inhalant (house dust mite, pollens, moulds, animal dander, cockroach), food (egg, milk, peanut, tree nuts, soya, wheat, fish, shellfish, sesame), venom (honeybee, wasp), drug (limited value) and occupational (latex) allergens.

Allergic rhinitis affects 10 to 30% of adults, asthma 5 to 10%, and food allergy 2 to 4% of adults and 6 to 8% of young children. Component resolved diagnostics measure IgE to single allergen molecules (peanut Ara h 2, hazelnut Cor a 9, Der p 1 and Der p 2, Phl p 1, venom Ves v 5 and Api m 1), and distinguish true primary sensitisation from cross-reactivity (profilins, polcalcins, carbohydrate determinants) with better prediction of severe reactions.

Indications. Allergic rhinitis, conjunctivitis or asthma with uncertain triggers. Suspected IgE-mediated food allergy (urticaria, angio oedema, vomiting, wheeze or anaphylaxis within minutes to 2 hours). Systemic reaction to insect sting (before venom immunotherapy). Latex allergy. Eczema in infants with suspected food triggers.

Patients who cannot have skin prick tests (extensive dermatitis, dermographism, antihistamines that cannot be stopped, high anaphylaxis risk). Selection of allergens for immunotherapy. Not for chronic non-specific symptoms, fatigue or food intolerance, and not for delayed non-IgE reactions.

Specimen. Serum 1 mL, no fasting. Stable for days at 2 to 8 °C and longer frozen. Antihistamines, steroids and omalizumab do not usually change the in vitro result.

Reference values (ImmunoCAP).

Interpretation.

• Positive IgE with a convincing history of reaction: allergy confirmed.
• Positive IgE with no symptoms on exposure: sensitisation without clinical allergy (common, 30 to 50% of those with positive results to foods tolerate them).
• Negative IgE (under 0.10 kUA/L) with a typical history: allergy unlikely but not excluded, since negative predictive value is high (over 95% for most foods) but not absolute.
• Higher levels increase the probability of clinical reactivity but do not predict severity. Food specific cut offs with 95% positive predictive value are published for children (cow's milk about 15 kUA/L, egg white about 7, peanut about 14), and they differ by population, age and assay.
• Component testing: peanut Ara h 2 over 0.35 kUA/L suggests true peanut allergy with risk of systemic reaction, while isolated Ara h 8 or Bet v 1 homologues suggest pollen cross-reactive, mild oral symptoms. Cor a 9 and Cor a 14 predict hazelnut systemic risk, and Cor a 1 suggests birch pollen cross-reactivity.
• Venom: both honeybee and wasp positivity may reflect cross-reactive carbohydrate determinants, and components (Api m 1, Ves v 5) identify the true allergen.
• Sensitisation to house dust mite (Der p 1, Der p 2), grass (Phl p 1, Phl p 5), birch (Bet v 1), cat (Fel d 1) and moulds helps select immunotherapy.
• Rising levels over time in a child on avoidance suggest persistent allergy, while falling levels suggest developing tolerance and support supervised challenge.

Critical values and findings needing immediate action.

• High specific IgE to a food with a history of anaphylaxis: strict avoidance, adrenaline auto injector, written emergency plan and specialist referral.
• Positive venom IgE after systemic sting reaction: venom immunotherapy is indicated, and the baseline serum tryptase is checked for mastocytosis.
• Latex sensitisation in a healthcare worker or spina bifida patient: latex free environment.
• Positive IgE in a patient with unstable asthma and exposure to the allergen: optimise asthma control and remove exposure.

Interferences and pitfalls.

• Sensitisation is not allergy. Testing without a pre-test history causes over diagnosis and unnecessary food avoidance (malnutrition, anxiety, loss of tolerance).
• Large screening panels ("food mix" results) have low specificity, and individual or component tests chosen from the history are better.
• Cross-reactivity is common: birch pollen with apple, hazelnut and carrot; grass with peanut and wheat; latex with banana, avocado and kiwi; house dust mite with shrimp (tropomyosin); cat with pork (alpha gal is a separate cause of delayed meat allergy).
• Very high total IgE can cause false positive low level specific results, and interpretation of low specific IgE in the setting of very high total IgE needs care.
• Results from different manufacturers are not interchangeable.
• The oral food challenge is the reference test for food allergy, and is done under supervision with resuscitation facilities.
• Skin prick tests are more sensitive for inhalants and give results in minutes, while specific IgE is safer for severe past reactions.
• Drug allergy testing by specific IgE is limited to few drugs (penicillin, suxamethonium), with low sensitivity.
• Specific IgE is not useful for delayed or non-IgE reactions (food protein induced enterocolitis, coeliac disease, contact dermatitis).

Special groups.

• Infants and children: early introduction of allergenic foods is encouraged, and testing is done only when there is a reaction or severe eczema. Many children outgrow milk and egg allergy, while peanut and tree nut allergy persists in 80%.
• Pregnancy: testing is safe, and adrenaline use for anaphylaxis is not withheld.
• Older adults: levels are lower, and sensitisation is less frequent.
• Regions: helminth infection and parasitic exposure raise total and cross-reactive IgE, giving many clinically irrelevant positives. Regional pollen and mould profiles shape panel choice.

Follow up tests. Skin prick tests, component resolved IgE (Ara h 2, Cor a 9, Der p 1 and 2, Phl p 1, Api m 1, Ves v 5), basophil activation test where available, supervised oral food challenge, total IgE, serum tryptase, full blood count with eosinophils, spirometry with reversibility, FeNO, nasal endoscopy, and patch testing for contact dermatitis.