Synonyms |
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RAST |
Clinical
Indications |
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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:
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History
of previous anaphylaxis following antigen exposure
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Dermatographism
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Extensive
eczema
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Very
young children
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Suspected
sensitivities to some foods
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Bee
and wasp venom sensitivity
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Patients
receiving anti-histamine therapy
-
Suspected
occupational allergy
-
Suspected
latex allergy (latex
allergy testing guidelines available)
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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 |
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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 |
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Request
Form |
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Combined Pathology Blood form
(Yellow/Black or Blue for GP's) and Allergy
questionnaire |
Availability |
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Analysed by referral
laboratory if specific criteria met. |
Specific
Criteria |
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Requests
must be accompanied by a completed Allergy
questionnaire which has been authorised by a Consultant. |
Turnaround
Time |
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3
weeks |
Specimen |
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Serum
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Container |
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Yellow top (SST) tube |
Lab.
Handling |
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Aliquot and store at 4C. (RAST & send; NOIGE & save)
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Causes
for Rejection |
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Unlabelled sample.
Not meeting specific
criteria for analysis.
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Interpretation |
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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. |