LABORATORY TESTS FOR ALLERGY
Laboratory tests for allergy include the eosinophil test. Eosinophils are white blood cells always present at the place where an allergic reaction takes place. Samples of blood, or of sputum, or of secretions from nose or eye, are taken. The cells are stained with a red dye, eosin, and are counted under a microscope. A high count indicates an allergic reaction is taking place, but it can be an indication of other diseases as well, and it can also be found in symptom-free individuals. Steroid tablets suppress the level of eosinophils and can cause misleading results. The eosinophil test cannot identify allergy to specific sub-stances.
A useful but expensive laboratory test for allergy is the radio-aller-gosorbent test, or RAST test. It can measure the levels of IgE antibodies in the blood specific to a particular allergen, such as pollens, house dust mites or food proteins. The blood sample is passed over an extract of the allergen attached to an inert substance. The IgE antibodies will bind to the allergen if they are present in the blood, as during an allergic reaction. Then another liquid, containing anti-IgE antibodies marked with radioactivity or colour, is passed over the sample. These will adhere to any IgE bound to the allergen, or will simply wash away if none is there. The level of IgE in the sample can then be taken by measuring the level of the marked anti-IgE that does not adhere.
The RAST test is more helpful than skin tests in cases of food allergy, and its results are not influenced by medication. However, results can vary – some people allergic to seasonal allergens, such as moulds or pollens, will have negative results outside the season, but positive results when exposed to their allergen. Similarly, if you have not eaten a problem food for some time, you may get a negative result, whereas you could get a positive result if you eat it regularly.
A modified version of the RAST test can identify false food allergy, as well as true allergy.
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