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PTD -  158

Test Directory -  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 (latex allergy testing guidelines available)

Patient history, examination and, if necessary' a diary are important for correct interpretation of results e.g food and symptom diary

Clinical scenario
Asthma - perennial
Asthma - perennial but worse at night
Seasonal rhinitis or conjunctivitis
Eczema
Peanut allergy
Insect venom anaphylaxis
Suggested allergens
House Dust Mite (HDM), cat, dog
HDM, cat, dog, mixed feathers

HDM, cat, dog, mixed grass pollens
HDM, mixed foods
Peanut, mixed nuts
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.

A pollen count calendar is available which shows an average pattern of release of the main allergenic pollen throughout the year.

Request Form 

Combined Pathology Blood form (Yellow/Black or Blue for GP's) and Allergy questionnaire

Availability

Referred Test: Analysed by Biochemistry, Southend Hospital if specific criteria met.

Specific Criteria

Requests must be accompanied by a completed Allergy questionnaire which has been authorised by a Consultant or GP.

Patient preparation None required

Turnaround Time

2 weeks

Specimen

Serum

Volume

A minimum of 1-2 mL is required depending on number of IgE specific allergen tests requested. If more than 2 IgE specific tests required

Container

Yellow top (SST) tube or

Paediatric orange top (lithium-heparin). If more than 2 IgE specific tests required then please send 2 orange top bottles.

Causes for Rejection

Unlabelled sample. Not meeting specific criteria for analysis. 
Reference range See interpretation

Interpretation

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.

Lab. Handling

Processing: Aliquot (MPA) and store at 4C in separating fridge (CB39). Referral:  IgE and send or NOIGE and save in separating freezer (CB40) at
-20C, saved frozen rack.

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Version 1.1 / December 2014                                                                                                  Approved by: Consultant Biochemist