Nonspecific Allergy Tests
Posted by admin on June 3rd, 2009
DIAGNOSTIC TESTS
Many pediatricians believe in the need for a screening test for allergy. Blood eosinophilia and total serum IgE levels have been proposed as screening tests, but they have relatively low sensitivity and should be used selectively (Table 3). The nasal secretions or sputum of patients who have a respiratory allergy contain increased numbers of eosinophils, which forms the basis of a useful nonspecific test, although not one that will identify any specific allergen etiology. Eosinophilia may not be present in patients who have not been exposed to allergens recently or who have a superimposed upper respiratory tract infection. Both systemic and inhaled steroids can reduce eosinophilia
in secretions significantly; antihistamines have no direct effect on eosinophils.
Why do people get Allergies
The usefulness of nasal eosinophilia as a diagnostic test depends in large part on the technique used to obtain the specimens to prepare the slides for examination. Patients should expel nasal secretions onto wax paper or parafilm; secretions then are spread on a microscope slide, stained, and eosinophils counted under a microscope. It is difficult to quantify nasal eosinophilia accurately, although a finding of more than 3% eosinophils on stained smear of expelled nasal or bronchial secretions is considered increased. Because cotton or nylon nasal swabs trap secretions, they are not recommended for collecting secretions, except in the young child who will not or cannot expel secretions by blowing the nose. Peripheral blood eosinophilia is observed in allergic asthma but less commonly in allergic rhinitis. Blood eosinophilia is more frequent in atopic dermatitis and other conditions, such as parasite infection.
Total serum IgE is elevated in about 60% of patients who have allergic asthma but only in 30% of those who have allergic rhinitis.
Unfortunately, commercial laboratories have promoted tests of total serum IgE excessively, but its usefulness in screening for allergy is limited to positive tests only because more than 60% of patients who have nasal allergy will have normal levels of total serum IgE.
Tags: Allergies, allergy, allergy tests