Diagnosing allergic diseases

Key Points

The pediatrician often is faced with having to diagnose disorders such as asthma, allergic rhinitis, atopic dermatitis, and food allergies. The history and physical examination are the primary means to confirm whether symptoms mean an Allergy. However, serologic tests can be helpful in confirming an allergy and identifying potential triggers.

In vitro tests for allergen-specific immunoglobulin E (IgE) can assist in the diagnosis and initial management of atopic disorders. A key point is that although these tests identify the presence of allergen-specific IgE, they do not, in isolation, provide a diagnosis. It is important to select and interpret these tests in the context of a medical history.

Atopic diseases and the role of allergen-specific IgE testing

DISTINGUISHING SYMPTOMS

A major concern for the pediatrician is to differentiate symptoms attributable to allergy from those that are not allergic and to identify allergic triggers when allergy is a likely concern. The most important means to approach this diagnosis is by history.

MECHANISM OF IGE TESTS

Tests for allergen-specific IgE are helpful in identifying an atopic disposition and for confirming specific causes of atopic disease but only when test selection and interpretation are done in the context of a thorough history. Allergen-specific IgE, produced by plasma cells, may be detected in the circulation by a serum test. These IgE antibodies bind to high-affinity IgE receptors on mast cells and basophils.

Allergy skin tests (not discussed extensively here) allow detection of allergen-specific antibodies by introducing an allergen epicutaneously using a probe and with little discomfort. The tested allergen binds the specific IgE on the mast cells, triggering a cascade that results in the release of mediators such as histamine.

A positive response results in a wheal and flare that can be measured. Both the serum IgE test and the skin test are very sensitive for detecting allergen-specific IgE, a state of sensitization. Each has advantages and disadvantages.

The skin tests generally are less costly, are sometimes more sensitive, and give immediate results.

The serum tests are more widely available, less subject to operator dependence, and not affected by patient skin conditions or use of antihistamines. However, the tests alone, without consideration of the medical history, do not indicate clinical allergy because many persons with allergen-specific IgE do not experience clinical symptoms on exposure to the allergen.

There are a number of reasons to pinpoint specific allergies. Doing so may identify children at increased risk for developing additional atopic diseases.¹² Some studies have indicated that early intervention, for example with allergen immunotherapy for those suffering from allergic rhinitis, may prevent the development of further disease progression to asthma.¹³ Moreover, identification of allergen-specific IgE can provide information about specific triggers that can be avoided, resulting in reduced symptoms.