Investigation of Allergy
Specific IgE antibody
Laboratory tests for diagnosis of allergy can be extremely useful but often are not. The laboratory has therefore developed a policy which it is hoped will provide the best possible cost-effective service.
The aim must be the selective testing for specific IgE antibodies tailored to the clinical history. All encompassing 'blanket' screens for specific IgE antibodies (e.g RAST screen, allergy screen) are not acceptable. Requests for IgE determinations against specific allergens will be carried out as requested.
It is possible to test for specific IgE against a very large number (100s) of allergens. The tests are relatively expensive and it is important to be clear what useful information will be obtained from testing against a large number of allergens. Ideally we are seeking to provide the patient with information which might allow them to avoid potentially harmful allergens. There is no substitute for a good clinical history. In order to avoid non-specific requests, we actively discourage requests for RAST without specific allergens being requested.
Requests for RAST with no clinical details will be screened against a mixture of the most common inhaled allergens only (inhaled mix). If clinical details suggest food allergy, samples will be screened against a mixture of common food allergens (common food mix). A negative result with these screening tests will almost certainly rule out allergy. In the event of a positive result further tests will not be performed without discussion with one of the consultant medical or scientific staff. An atopic individual is likely to have specific IgE to many, if not all, of the allergens in the inhaled allergen mix.
Since there is a considerable body of evidence to suggest that total IgE concentration is a poor guide to allergic status total IgE will not be measured unless specifically requested. The most widely used laboratory tests in allergy are the quantitation of total plasma IgE and tests for the measuremt of specific IgEs recognising a wide range of allergens including inhaled allergens, food allergens, drugs, venoms and occupational allergens including latex.
Total IgE is usually raised in allergic individuals. Levels are very high in atopic eczema and bronchopulmonary aspergillosis. However, total IgE may be also raised in parasitic infections, lymphoma, some liver diseases and some autoimmune diseases such as Churg-Strauss syndrome. The highest levels are seen in hyper-IgE (Job) syndrome.
In allergic diseases, high levels are associated with atopy and multiple reaction to a wide range of unrelated antigens. The level of IgE does not correlate well with severity of symptoms. There is a wide body of evidence to show that IgE concentration is a poor guide to allergic status. The normal range is very wide; normal levels of IgE can be associated with very high levels of IgE to a single allergen. The laboratory will no longer measure total IgE unless specifically requested.