IgE (Total & Specific Allergy) Blood Test

What is IgE?

Immunoglobulin E (IgE) is an antibody class produced by plasma cells (immune cells) in response to allergens: substances the immune system mistakenly identifies as threats, such as pollen, pet dander, dust mites, foods and insect stings. IgE binds to mast cells in tissues. When the allergen is encountered again, IgE triggers mast cell degranulation, the release of histamine and other chemicals that cause allergy symptoms. Total serum IgE gives a general picture; specific IgE tests (also called RAST or ImmunoCAP) identify which specific allergens you are sensitised to.

Total IgE normal range

Causes of high total IgE

Specific IgE testing (RAST / ImmunoCAP)

Specific IgE tests measure IgE antibodies against individual allergens in a blood sample. Common panels include: respiratory (dust mite, grass pollen, cat/dog dander, mould), food (peanut, tree nuts, milk, egg, wheat, shellfish), insect sting (bee, wasp) and latex. Results are reported in classes from 0 (not detected) to 6 (very high). Class ≥2 is typically considered clinically relevant.

Questions to ask your doctor

• Should I have a specific IgE panel to identify my trigger allergens?
• Do I need a skin prick test as well?
• Could a parasitic infection be causing my high IgE?
• Am I a candidate for immunotherapy (allergy shots or sublingual drops)?
• Should I carry an adrenaline (epinephrine) auto-injector?

References

Sources cited on this page. PubMed links open the original abstract.

1. Johansson SGO, Bieber T, Dahl R, et al. Revised nomenclature for allergy for global use. J Allergy Clin Immunol. 2004;113(5):832–836. PMID 15131563 · doi:10.1016/j.jaci.2003.12.591

Total IgE versus specific IgE – what each test tells you

There are two distinct IgE measurements, and they answer different clinical questions:

• Total IgE: Measures all IgE antibodies in the blood regardless of what they target. A raised total IgE (above 100–120 kU/L in adults) suggests an atopic constitution (tendency to allergic conditions) or parasitic infection, but does not identify which allergen is responsible. Extremely high total IgE (above 2000 kU/L) can indicate allergic bronchopulmonary aspergillosis, hyper-IgE syndrome, or tropical parasitic infection.
• Specific IgE (sIgE, formerly RAST): Measures IgE antibodies directed against a particular allergen – grass pollen, cat dander, peanut protein, house dust mite, cow's milk protein, etc. A raised sIgE to a specific allergen means the immune system has been sensitised to it, but sensitisation does not always equate to clinical allergy. Positive results must be interpreted alongside the patient's history.

Modern specific IgE panels (component-resolved diagnostics) go further – testing against individual protein components within an allergen. For peanut allergy, for example, Ara h 2-specific IgE is a far stronger predictor of severe systemic reactions than a positive result to whole peanut extract.

Skin prick testing versus blood sIgE – when to use which

Both skin prick testing (SPT) and specific IgE blood tests detect IgE-mediated sensitisation, but they differ in practical terms:

• Sensitivity: SPT is generally slightly more sensitive for inhalant allergens (pollens, animal dander). Blood sIgE is equally or more sensitive for food allergens.
• Antihistamine interference: SPT cannot be performed while the patient is taking antihistamines (they suppress the skin reaction). Blood sIgE can be measured at any time regardless of antihistamine use.
• Safety: SPT carries a very small risk of systemic reaction in highly sensitised patients. Blood tests carry no such risk.
• Immediate results: SPT results are available in 15–20 minutes. Blood sIgE results take 24–48 hours.

NICE guidelines (CG116) recommend that allergy testing should always be interpreted by a clinician with allergy training alongside a detailed history – neither test alone is sufficient to diagnose clinical allergy.

Conditions where IgE testing does NOT help

Not all adverse reactions to foods or substances are IgE-mediated. IgE testing is unhelpful and should not be ordered for:

• Food intolerances (e.g., lactose intolerance, FODMAP sensitivity) – these are enzyme deficiencies or gut fermentation issues, not immune reactions.
• Non-IgE-mediated food allergies (e.g., food protein-induced enterocolitis syndrome – FPIES) – mediated by T cells, not IgE.
• Contact dermatitis (e.g., nickel allergy) – mediated by T cells; diagnosed by patch testing.
• Drug intolerances that are pharmacological rather than immunological.

The widespread ordering of IgE panels for unexplained fatigue, joint pains, or non-specific symptoms is not supported by evidence and leads to unnecessary dietary restriction.