pathologyhandbook
Royal Berkshire NHS Foundation Trust
Allergy Testing
Specific IgE testing is of value where skin testing is difficult to perform, unreliable, or contraindicated, i.e.:-
1. in very young children
2. in patients with severe/extensive eczema or dermatographism
3. in patients taking anti-histamines which cannot be stopped
4. in patients in whom there is a significant risk of an anaphylactic response.
The use of “RAST” testing must be carefully considered and is not a substitute for careful clinical assessment.
We offer a total IgE estimation followed by a range of allergen specific IgE tests. In the first instance we suggest up to a maximum of 5 allergens per sample. The range of allergens available in house is given in Table 1. This table reflects the position at the time of writing but this will change as the service develops. Allergen specific IgE requests that are clinically indicated by the history and not contained in this list are referred on to a specialist laboratory. A good clinical history is of paramount importance and should guide the requesting of this service. Requesting of multiple allergens as a blanket screen is very rarely helpful.
We will use the total IgE to screen for atopy and if the clinical symptoms are non-specific and the total IgE is very low, then we may not proceed with the allergen specific IgE requests. (Reference: Sinclair D, Peters, SA 2004: The predictive value of total serum IgE for a positive allergen specific IgE result in children. J Clin Pathol. 2004; 57:956-9)
If however, the symptoms are suggestive of a Type I allergic response to a single allergen, then we will not be influenced by the total IgE concentration and we will look for allergen specific IgE. Perennial rhinitis may be investigated with IgE to house dust mite and cat or dog, presentations in spring might also include tree pollen, summer presentations might include grass pollen and patients presenting in autumn might usefully be tested for IgE to Aspergillus fumigatus. Rhinitis affected patients presenting with symptoms that are worst first thing in the morning might be reacting to feathers in pillows. Hence, there is an absolute requirement for full clinical data to support each request. However, in general terms, we discourage screening policies using a fixed panel of allergens and each request is dealt with on its own individual merits. Similarly, with the exception of grass, tree, mould and weed pollens, we do not offer “allergen screening panels” but require that you specify the relevant individual allergens e.g. we will not offer a “food panel” or “inhalant panel”. Requests for “RAST testing” without specifying any allergens may be returned to the clinician and the sample stored pending further the receipt of relevant clinical information.
It should always be borne in mind that the presence of allergen specific IgE does not always mean that the allergen in question is responsible for the patient’s symptoms and the absence of allergen specific IgE does not exclude an adverse reaction.
Grade 0 = <0.35
Grade 1 = 0.35-0.70
Grade 2 = 0.70 - 3.50
Grade 3 = 3.50 - 17.5
Grade 4 = 17.5 - 50
Grade 5 = 50-100
Grade 6 = >100
Allergen-specific IgE can be found without clinical reactions especially in atopic subjects. They are NOT proof of allergy and are not useful for screening. Clinical assessment is required to interpret such results.
Table 1
| General Foods |
| Milk |
| Wheat |
| Tomato |
| Orange |
| Strawberry |
| Apple |
| Cheddar Type Cheese |
| Chicken Meat |
| Kiwi |
| Lemon |
| Egg |
| Lime |
| Chickpea |
| Nuts and Seeds |
| Sesame |
| Peanut |
| Soya Bean |
| Hazelnut |
| Brazil Nut |
| Almond |
| Coconut |
| Pecan Nut |
| Cashew Nut |
| Pistachio Nut |
| Pine Nut (Pignoles) |
| Walnut |
| Sea Foods |
| Codfish |
| Crab |
| Shrimp/Prawn |
| Blue Mussel |
| Tuna |
| Lobster |
| Squid |
| Animal Danders and Epitheilia |
| Cat |
| Horse |
| Dog |
| Guinea Pig |
| Rabbit |
| Insect Venoms |
| Honey Bee |
| Wasp |
| Bumble Bee |
| Miscellaneous |
| House Dust Mite |
| Latex |
| Aspergillus fumigatus |
| Allergen Panels |
| Grass Pollens |
| Tree Pollens |
| Weed Pollens |
| Moulds |