Trip Answers

Question answered:09/03/07 Warning! this question is over two years old.

We undertook a literature search in the TRIP and Medline databases but found no specific information on allergy testing in relation to dogs. However, we did locate several resources discussing methods of allergy testing in general which should be relevant.

 

The Royal College of Physicians issued a report on the management of allergy and provision of allergy services which discusses the role of domestic pets as environmental allergens noting:

 

“Surprisingly, many people who are sensitive to pet allergens still choose to keep pets in their homes. Allergic reactions to cats, dogs and horses occur frequently, and sensitisation to one animal is often accompanied by sensitisation to other related or non-related species…” p. 39

 

Chapter 9 of the report discusses the diagnosis of allergy:

 

Skin prick tests

 

“Skin prick or laboratory tests can help to confirm the clinical diagnosis in allergy. However, these tests are of limited value without a detailed clinical history.

 

Simple allergies such as hay fever do not require confirmatory diagnostic tests (the exception being when immunotherapy is to be given). In other conditions it is essential to demonstrate specific IgE antibody, and this can be done by a skin prick test (SPT) or by measuring specific IgE in the serum. These are different ways of measuring the same antibody response, but the SPT is superior. Confirmation of the diagnosis is required: in any severe allergy; where avoidance measures will be instituted (eg for house dust mite, food, drug or latex allergy); if immunotherapy is required; and in other conditions if the diagnosis is not clear from the history.

 

Intradermal tests

 

Suspected allergens can also be injected directly into the skin (intradermal (ID) tests), but they can produce painful reactions, are technically more difficult, and sometimes give misleading results because the needle itself can cause sufficient damage to produce a false-positive response.

 

The measurement of allergen-specific IgE in serum has improved markedly over the last decade. Specific IgE against a wide range of well validated allergens is now available.

 

Measurement of allergen-specific IgE is of value when facilities for SPT are unavailable or when SPT is otherwise contradicted, eg atopic dermatitis involving areas where SPTs are performed, or in patients taking H1-antihistamine. However, for other allergens, serum-specific IgE tests are either (i) not available, or (ii) available but of poor quality, eg fruits and vegetables, and for some of these SPTs can be performed using the prick–prick test technique.

 

Special challenge (provocation) tests may be needed to make the diagnosis of allergy, for example to foods and drugs, when often there are no validated skin tests or laboratory tests and/or the mechanism does not involve IgE. Double-blind placebo-controlled tests can often identify or disprove food intolerance by giving suspected foods or food additives in disguised forms. Allergen inhalation tests can confirm suspected causes of occupational asthma.

 

SPTs are cheap and easy to perform once staff are trained, whereas serum IgE tests are more costly but still relatively cheap. Challenge tests are a day-case procedure in a specialist allergy unit with facilities and expertise for treating anaphylaxis. Patch testing for contact dermatitis is valuable and is done primarily in dermatology departments. Lymphocyte transformation tests and basophil histamine release or flow cytometry analysis of cell surface ‘activation’ markers have been reported to be helpful for diagnosing drug allergies, but these tests have not yet been validated.”

 

Table 9.1 focuses on the range of diagnostic tests available and outlines the rationale, indications, interpretation and limitations of each test (see pages 74-75), and this is followed by a second table (9.2) listing and discussing alternative tests of no proven value in allergy diagnosis.

 

A second publication by the Royal College of Physicians, a patient guide to allergy and allergy tests, may also be useful. [2]

 

In November 2006 the NLH Q & A Service answered a question on 'alternative' tests for diagnosing allergieswhich may also be of interest, and thismay be read at http://www.clinicalanswers.nhs.uk/index.cfm?question=4341.

References
1. Royal College of Physicians. Allergy: the unmet need. June 2003. (http://www.rcplondon.ac.uk/pubs/books/allergy/allergy.pdf).
2. Royal College of Physicians. Allergy and allergy tests. June 2002 (revised 2005). (http://www.rcpath.org/resources/pdf/allergy_doc.pdf).

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