Pathology Handbook

Allergen Specific IgE

Synonyms

RAST

Clinical Indications

The demonstration of allergen specific IgE (Ag-IgE) in serum provides objective evidence for presence of specific antigens which may be responsible for allergic problems.
In most cases of moderate and severe hypersensitivity the diagnosis and identification of relevant antigens can be made on clinical grounds alone. Ag-IgE tests are of value in situations where genuine clinical doubt exists. In patients older than 8 years with a low total IgE level (<25 kU/L), Ag-IgE is rarely found even to single antigens. 

Specific clinical situations where Ag-IgE may be considered:

  • History of previous anaphylaxis following antigen exposure

  • Dermatographism

  • Extensive eczema

  • Very young children

  • Suspected sensitivities to some foods

  • Bee and wasp venom sensitivity

  • Patients receiving anti-histamine therapy

  • Suspected occupational allergy

  • Suspected latex allergy

Clinical scenario

Suggested allergens

Asthma - perennial

House Dust Mite (HDM), cat, dog

Asthma - perennial but worse at night

HDM, cat, dog, mixed feathers

Asthma - exacerbation

as above plus seasonal allergens

Seasonal rhinitis or conjunctivitis

HDM, cat, dog, seasonal pollens/allergens

Eczema in young children

Mixed foods

Eczema in adults

Screen depends on clinical information given

Nut allergies

Nut screen

Insect venom anaphylaxis

Bee and wasp venom

Screening may be done using standardised allergen mixes e.g. Food mix - cow's milk, egg white, fish (cod), wheat, peanut and soya; Inhalant allergen mix - grasses, trees, moulds, HDM, pets

Click to read leaflet produced by the Royal College of Pathologists. . 

Request Form 

Request on ICE

Availability

On request

Specific Criteria

The request MUST contain information about which allergens trigger an attack

Turnaround Time

1 week

Specimen

Serum in gold-top vacutainer

Container

Vacutainer gold top

Causes for Rejection

Unlabelled or incorrectly labelled sample; not meeting specific criteria for analysis or in wrong sample container

Interpretation

See laboratory report.

The finding of a positive Ag-IgE test does not indicate that any particular antigen is responsible for the patient's symptoms and interpretation must be in the context of a full allergic history.

Moreover, the absence of Ag-IgE in serum does not exclude IgE involvement since, particularly in allergic rhinitis, local synthesis and mast cell sensitisation of IgE can occur in the absence of detectable serum Ag-IgE. 

Please note that the test will be negative in cases of non-IgE mediated hypersensitivity.