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Guide to Allergy Testing

Allergy: What is it?

Allergic disorders result from an inappropriate, usually IgE-mediated, immune response upon exposure to either environmental or food allergens. Common manifestations of allergy include rhinoconjunctivitis, asthma, eczema, acute urticaria and anaphylaxis.

Disorders, such as chronic urticaria, hereditary angioedema and T-cell contact dermatitis (metal allergy), while clinically similar in some ways, are not IgE-mediated. Allergic disease manifests in different ways through life and the likely causative agents can also change with age (see Table 1).

Tests used in the diagnosis of IgE-mediated allergy

Total IgE

Higher levels of total IgE are often found in patients with allergic conditions. However, normal total IgE does not exclude allergy.

Total IgE is also elevated in other conditions including parasitic infections and allergic bronchopulmonary aspergillosis. It is used increasingly in determining anti-IgE therapy in moderate to severe asthmatics.

Allergen-specific IgE

Allergen-specific IgE can be detected for a large variety of allergens. The presence of a specific IgE to allergen can suggest allergic disease and is detected via a blood test (RAST or radioallergosorbent test) or skin prick test. RAST tests detect many of the different proteins within an individual allergen.

Recombinant allergen testing

Of the many proteins within a substance, only a few may cause allergic symptoms. Recombinant allergen testing looks for specific characterised protein within an allergen.

Interpretation of RAST tests

The presence of detectable specific IgE to an allergen does not confirm the patient is allergic to that substance. All results must be interpreted in conjunction with the clinical history of the patient.

Low levels of detectable specific IgE can confirm the presence of allergy in the right clinical context. RAST testing aids in the assessment of, and identification of allergic sensitisation, but is not to be used alone as the deciding factor for inclusion or exclusion of allergy.

As the level of specific IgE increases, the likelihood of clinical relevance also increases. As shown in Table 2, different allergens have different specific IgE level cutoffs at which serious allergy is >95% likely (positive predictive value or PPV). The range of values is vastly different between allergens and is affected by age and also by geographic region.

Table 2 defines levels at which exposure, or a challenge, would be highly hazardous for a patient. Importantly, many patients could have serious reactions at much lower levels.

 

TABLE 1. Age-related allergens

AgeAllergic condition and allergen
<2years Eczema and anaphylaxis: Typically food
2-10 yearsEczema: Typically environmental and some foods
Allergic rhinoconjunctivitis: Typically environmental and some foods
Asthma: Typically environmental, food if acute episode is associated with eating
>10 years Asthma and allergic rhinoconjunctivitis: Typically environmental Anaphylaxis: Food, medications, stinging insects

 

TABLE 2. Allergen specific IgE - Specific IgE levels conferring high risk

AllergenAgeIgE levelPPV
Egg <2 years2 kU/L
~95%
≥2 years7kU/L~98%
Fish20kU/L~100%
Milk<2 years
5kU/L
~95%
≥2 years15kU/L~95%
Peanut14kU/L~100%
Prawn>0.35kU/L~85%

RAST tests

RAST tests are available for a range of allergens, however Medicare criteria limits rebates based on the number, type and frequency of tests.

Medicare Australia limits rebates for RAST tests to a maximum of four specific allergens and/or mixes per pathology request and a maximum of four RAST test episodes per year.

When ordering RAST tests, it is advisable to include allergens the patient feels are relevant and those likely for the clinical scenario. For common clinical scenarios we recommend the following:

Childhood eczema

Age <2 years: Milk, Egg, Wheat, Peanut

Age >2 years: Milk, Egg, Peanut, Dust mite

Additional allergens or an extended RAST combined allergy panel may be ordered.

Asthma and allergic rhinoconjunctivitis

Dust mite, Grass mix, Animal dander

Additional allergens may be ordered or substituted if relevant (e.g. cat dander instead of animal dander). An extended RAST inhalant panel is also available.

Default panel if no allergens are specified and no clinical notes are provided

Age <5 years: Dust mite, Grass mix, Food Mix

Age >5 years: Dust mite, Grass mix, Animal Mix

Anaphylaxis

Anaphylaxis is a severe life-threatening allergic reaction. It is recommended these patients require specialist assessment by a clinical immunologist or allergist.

Initial testing should look for the causative allergen if possible. It is important to note that a negative RAST test does not exclude the allergen tested.

RAST testing recommendations ­

Extended RAST panels

Extended RAST panels have been developed to represent the common allergens encountered clinically in practice. They are particularly relevant in our geographic region and replace the skin prick test panel which is no longer available. Additional allergens may also be requested. All results must be interpreted in conjunction with the patient’s clinical history.

Extended RAST Food Panel ­

Extended RAST Nut Allergy Panel ­

Extended RAST Combined Allergy Panel ­

Extended RAST Inhalant Panel ­

Recombinant allergens

Omega-5 gliadin ­

Alpha-gal (mammalian meat allergy) ­

Peanut Allergy Risk Assessment ­

How to order allergy tests

RAST tests – standard panels Medicare Australia limits rebates for RAST tests to a maximum of four specific allergens and/or mixes per pathology request and a maximum of four RAST test episodes per year.

Extended RAST tests (Medicare rebate + $120* per panel)

Please note, extended RAST panels are not bulk billed.

Recombinant allergen tests (Medicare rebate + $60* each)

 

General Practice Pathology is a regular column each authored by an Australian expert pathologist on a topic of particular relevance and interest to practising GPs. The authors provide this editorial free of charge as part of an educational initiative developed and coordinated by Sonic Pathology.