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Diagnostic Tests
Nonspecific
Allergy 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 . 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.
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.
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