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AIlergy Testing - Specific IgE Reporting Changes

(November 2007)

(Download   Allergy Testing - Specific IgE Reporting Changes.pdf)

In line with national and international guidelines HAPS has recently changed the format for reporting Allergen Specific IgE results. The method for performing these tests remains unchanged (ImmunoCAP^TM).

Laboratory tests for the measurement of IgE antibodies to specific allergens have been available for 40 years and are often still referred to as RAST tests after the initial technology employed. Traditional RAST testing was semi-quantitative and several scoring methods were developed to organise results into classes which are still used today. Table 1 below outlines the comparison between the classification previously used and kU/L.

A list of allergens routinely available can be found on the HAPS website Handbook section (Handbook). Other allergens may be available on discussion with the laboratory.

Non specific requests for ‘RAST’, ‘Allergy tests’ or ‘Allergen specific IgE’ will be tested for House dust mite and grass mix only, and food related requests for staple food mix only.

Hints for best practice

• Use single allergens where possible

Allergen mixes have limited utility except to ‘rule out’ allergic disease in patients with a low pre-test probability of allergic disease. Mixes of seasonal allergens such as grass, weed and tree pollens may be of use in situations where skin prick testing is not possible. Given the increased rate of false negative results with allergen mixes appropriate selection of specific allergens provides more useful information.

• Specific requests addressing the clinical question help everyone

A few select questions will identify allergens of interest, eg. Are the symptoms persistent and/or occur indoors, implying HDM, animal dander, cockroach and mould, or intermittent and/or occur outdoors suggesting pollen allergy. Table 2 below outlines several testing scenarios.

• When to arrange a skin prick test

Skin prick testing (SPT) remains the “gold standard” for detection of allergen specific IgE. It is recommended that SPT’s are performed by appropriately trained medical practitioners with resuscitation facilities available as the procedure requires experience for accurate interpretation and rarely may induce anaphylaxis. Table 3 below summarises advantages and disadvantages of in vitro methods for detecting allergen-specific IgE.

Interpreting test results

A number of studies have been performed to evaluate the clinical performance of ImmunoCAP specific IgE tests. Allergens (especially food allergens) are relatively labile and can degrade.

• Sensitivity of allergen specific IgE = 84-95%
• 5-16% false negative rate
• Specificity of allergen specific IgE = 85-94%

A negative allergen Specific IgE in the presence of a strong history of an allergic response therefore requires further investigation (skin testing +/- challenge). It is also important to remember that not all sensitised individuals (i.e. those with detectable allergen specific IgE either by SPT or blood tests) have clinical symptoms.

Blood specific IgE testing can be difficult to interpret in patients who have very high levels of total IgE (> 1000 kU/L) (eg, patients with eczema), as they may have low-grade reactions to many allergens. However, total IgE levels are not required for the assessment of most patients.

A number of studies have now been performed to assess the role of the absolute value of Allergen Specific IgE for predicting the potential, presence or severity of allergic disease, particularly in the paediatric population.

• Inhalant allergens - no consistent relationship has been found.
• Food allergens - Studies by Sampson³ et al have shown that the quantity of specific IgE to certain foods can accurately determine the patients current clinical sensitivity. The probability curves produced outlined IgE thresholds for provocative testing below which there is a 95% probability that a food challenge will be negative and also upper thresholds at which there is a 95% probability that it will be positive. Such results are being used to identify which children are most suitable for food challenges. The current decision thresholds are summarised in Table 4 below (It is important to note that these results were found in a highly selected US paediatric population. Subsequent studies have generated different predictive values).

Currently there are no recommendations for the interpretation of absolute values for the adult population.  

¥ Tentative values

Key points

• HAPS is now reporting Allergen Specific IgE results in kU/L.
• Request specific allergens appropriate to the clinical problem.
• Interpret results in light of patient’s history as false positives and negatives do occur.
• Quantitative values of allergen specific IgE for certain foods may predict the outcome of a food challenge in select children. No such values exist for adults or inhalant allergens at this time.

Costs of testing

Testing for allergen specific IgE is covered by the Medicare Benefits Schedule (Item 71079), for Medicare Eligible patients to a maximum of four episodes in a 12 month period.

Testing frequency

Testing for allergen specific IgE is carried out 3 times / weekly, total IgE twice / weekly and tryptase twice / weekly.

Sample required

One serum tube (red top plain) is required for allergen specific IgE testing.

References

1. Mallon D, Laboratory Tests in the diagnosis of Allergic disease. http://www.allergy.org.au/aer/infobulletins/Laboratory_Tests.htm
2. Paganelli, R et al, Specific IgE antibodies in the diagnosis of atopic disease: Clinical evaluation of a new invitro test system, UnicapTM, in six European allergy clinics. Allergy (1998), Vol 53 (8), pp 763 – 768
3. Sampson HA, Food Allergy. J Allergy Clin. Immunol. (2003), Vol 111, No.2: pp S540 – 547.

Author

This information sheet was written by Dr Kathryn Patchett. The Immunology Department of HAPS can be contacted by telephone (49214018) or facsimile (49214023).