Assays / Protein Reference Units / IgE - Allergen Specific

Clinical use:
The importance of the demonstration of Ag-IgE in the serum is that it provides objective evidence for the presence of a potentially inflammatory specific antibody in the individual and therefore, provides a possible explanation of the mechanism and the specific antigens responsible for allergic problems. It has the advantage of encouraging the clinician to consider the immunological responses involved and to design a more rational therapeutic approach in individual patients. The finding of a positive Ag-IgE test does not indicate that any particular antigen is responsible for the patient's symptoms and determination of the significance of any result can only be interpreted in the context of a full allergic history. Moreover, the absence of specific IgE in the serum does not exclude the possibility of IgE involvement since, particularly in allergic rhinitis, local synthesis and mast cell sensitisation by IgE can occur in the absence of detectable serum Ag-IgE. In these circumstances clinical problems can only be elucidated by local challenge testing.

The ease with which Ag-IgE tests can be performed must not be allowed to over-exaggerate their value in the assessment of allergic patients. When acute symptoms in an atopic patient are due to extrinsic agents, the agent(s) responsible can usually be determined more simply by clinical history and skin prick tests. Skin tests are easy to perform and are an inexpensive clinical test of hypersensitivity. The correlation between Ag-IgE and skin tests is very good in cases of moderate and severe hypersensitivity and in most such cases the diagnosis and identification of relevant antigens can be made on clinical grounds alone and often without recourse even to a serum total IgE determination.

Ag-IgE tests are of definite value in situations where genuine clinical doubt exists or where skin tests cannot be performed or are difficult to interpret. In this situation laboratory investigations may be indicated. In adults a total serum IgE should be performed first since an elevated concentration correlates well with the likelihood of the presence of Ag-IgE and if low (<25 kU/L) then Ag-IgE is rarely found even to single antigens. Patients with negative skin tests and abnormal total IgE (>100 kU/L) are also extremely unlikely to have Ag-IgE to antigens commonly tested and screening assays in this situation are of no value.

Clinical Situations Where Specific IgE Analysis 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 Penicillin sensitivity
• Suspected occupational allergy

In patients with rhinitis or asthma in whom the serum IgE is elevated and skin test results are equivocal or not obtainable then it is reasonable to examine serum for Ag-IgE against common inhalant allergens (Cat, House Dust Mite and Grass Pollen). In cases of asthma prevalent moulds should be added to the panel and any additional provoking factors obtained from the clinical history. Clinical studies have indicated that routine examination of serum from these patient groups for Ag-IgE against food antigens is not of clinical value.

Food antigens can give unreliable results in skin prick testing and since most cases of IgE-based food sensitivity occur in young children serum tests for Ag-IgE may provide a more acceptable approach. Determination of Ag-IgE against foods (Egg white, Cow's milk, Wheat and Soya) may be valuable in children with suspected sensitivities regardless of the total serum IgE concentration, and in adults with elevated or high normal (>100 kU/L) total serum IgE concentrations. High concentrations of Ag-IgE against a wide range of inhalant and food antigens are frequently found in patients with atopic eczema. The clinical significance of food sensitivities, particularly in adult eczema cases, is often unclear and the information usually adds little to the clinical management. Most patients with spontaneous urticaria, hives or angio-oedema have normal total IgE concentrations. Routine examination of serum for Ag-IgE is of little value in the absence of a history of specific provocation.

Determination of Ag-IgE is of particular value in suspected Bee and Wasp venom sensitivity and allergy to the Penicillins and other drugs or anaesthetic agents even in the context of a low or normal total serum IgE. Such sensitivities are not confined to atopic individuals and skin prick testing can be unreliable or frankly hazardous in this clinical situation where desensitisation may eventually be required. Ag-IgE is also the method of choice for investigation of severe reactions to peanut and latex where skin testing may be contraindicated.

The varied clinical circumstances in which Ag-IgE may be requested and the uncertainty surrounding the value of the investigations dictate that screening policies using a fixed panel of antigens are to be discouraged and each clinical request should be dealt with on its individual merits. Such a service can only provide informed results if requests are accompanied by full clinical details, including skin test results and an indication of likely antigens responsible for symptoms.

The attached table gives some general guidelines on allergen selection in the more common clinical scenarios. It should always be remembered that other allergens may be implicated and that all patients should be considered on an individual basis in the light of their clinical history.

Clinical scenario Suggested allergens:

• Asthma - perennial HDM*, cat, dog
• Asthma - perennial but worse at night HDM*, cat, dog, mixed feathers
• Seasonal rhinitis or conjunctivitis HDM*, cat, dog, mixed grass pollens
• Eczema HDM*, mixed foods
• Peanut allergy Peanut, mixed nuts
• Insect venom anaphylaxis Bee and wasp venom
• Penicillin allergy Penicillin G and penicillin V, ampicillin
• * HDM - House dust mite Dermatophagoides pteronysinus

Sample requirement: 2 mL serum.
Whilst this is an adequate sample volume for up to FIVE specific IgE assays, requests for further analyses should be accompanied by a proportionally greater volume.

Reference range:
Allergen specific IgE concentrations can be reported in grades (0-6) or in units. The unit is arbitrarily related to the total IgE International Reference Preparation (75/702).
The potential clinical significance of Grades 1-3 will vary with the allergen concerned. Grade 1 positive to mould or food allergens may be regarded as significant, but of doubtful significance in inhalant allergens.

Grade Units Interpretation

0	<0.35	Negative
1	0.35-0.7	Weak Positive
2	0.7-3.5	Positive
3	3.5-17.5	Positive
4	17.5-52.5	Strong Positive
5	52.5-100	Strong Positive
6	>100	Strong Positive

Available Allergen Specific IgE Assays
Complete lists of available allergens can be obtained from the individual PRU on request. If you have a particular query as to the availablity or relevance of any allergen, contact the PRU.

Centres offering this assay: Cardiff, St.George’s, Sheffield

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