The diagnostic tests are designed to determine the allergen/allergens to which the patient is sensitized so that proper recommendations for treatment (such as avoidance of allergen and hyposensitization) can be given.
History of food habits, drugs taken, insect bites, and the details with respect to earlier allergic attacks are essential to find the allergen responsible for allergy. In fact, the immunologist should do a persistent detective work to identify the culprit (allergen).
(For example. Reaction to drinking milk may be due to presence of penicillin in milk. A reaction to vaccine may be due to presence of penicillin in vaccine. A reaction to vaccine may be due to egg white because the vaccine might have been prepared by culturing the virus in eggs.)
1. In-Vivo Diagnostic Tests:
a. Skin test for IgE mediated reactions:
Many skin test methods are used to diagnose IgE mediated reactions. The skin test indicates the presence or absence of an immune response against a specific allergen. Small quantities (10-20 µ1) of test allergens are injected individually below the epidermis of the patient at different sites.
The injected sites are observed for the development of ‘wheel and flare’ reaction, which occurs within 15 to 20 minutes after the injection. The diameters of the wheel are measured. A wheel of >5 mm with accompanying erythema is considered to be positive (i.e. the patient is allergic to the injected allergen).
The injected allergen binds to mast cell-bound specific IgE in the skin and leads to release of mediators, especially the histamine. The histamine increases the vascular permeability so that the contents from blood vessels leak into the tissue resulting in the swollen wheal. Dilatation of the blood vessels around the area produces the diffuse red ‘flare’ seen around the wheal.
Skin testing with drugs is generally not recommended (though skin testing is mandatory for few drugs like penicillin). Most of the present day drugs are organic chemicals, and they cannot act as an immunogenic by themselves. But they can act as haptens by their ability to combine with some tissue protein. In such instances these drugs can cause allergic reactions.
But skin testing with these haptenic drugs may not give a positive result because most of these haptenic drugs have monovalent antigen (but activation of mast cells requires the binding by polyvalent antigens to mast cell-bound IgE). Hence a negative skin test with haptenic drugs does not rule out the anaphylactic nature of the person to that particular drug.
b. Broncho provocation test:
The patient inhales the aerosolized allergen extracts to provoke a bronchial reaction. Lung function tests are carried out before and after the inhalation of the allergen to assess the airway constricting response. This test is useful in allergic asthma and hypersensitive pneumonitis.
c. Oral provocation test:
Freeze-dried foods packed in large opaque gelatin capsules are swallowed by the patient. Then the patient is observed for the development of symptoms and signs of hypersensitive reactions.
d. Elimination of diet testing:
Based on patient’s history of allergic attacks one or more foods are eliminated from the patient’s diet for certain period. One of the suspected foods is then reintroduced into the patient’s diet and the patient is observed for the development of allergic symptoms and signs.
2. In-Vitro Diagnostic Tests:
a. Measurement of total serum IgE:
Total serum IgE level is correlated with allergic diseases in a general way. Elevated serum IgE levels are seen in allergic diseases.
b. Radio-allergosorbent test (RAST):
RAST test is done by allowing the patient’s serum to react with different pure allergens. RAST measures the quantities of specific IgE antibodies against various allergens in the serum of a patient. RAST tests to measure specific IgE antibodies to allergens such as dog dander, cat dander, dust mites, tree grass, and ragweed pollen are available.