Introduction

Allergy tests are used to determine the specific substances that cause an allergic reaction in an individual. They may also be used to determine if a group of symptoms is a true allergic reaction (involving antibodies and histamine release). Some food intolerances, in which there is an inability to digest a food because of a lack of appropriate enzymes, mimic allergies. Some drugs, such as aspirin, can cause allergy–like symptoms, but without the formation of antibodies or the release of histamine.

Skin–prick Test

Skin–prick tests are usually less important than the history in the diagnosis of allergic conditions, but they often provide useful confirmatory evidence of reaction to suspected allergens. They are simple to carry out and interpret, but they do not give absolute evidence of clinically relevant allergy. A patient with asthma may, for example, show positive skin–prick reactions to allergens, which do not trigger bronchospasm on direct challenge test with allergens, which give negative skin–prick results.

The Scratch Test

It is one of the most common methods in allergy testing and it involves placement of a small amount of suspected allergy–causing substances (allergens) on the skin (usually the forearm, upper arm or the back), and then scratching or pricking the skin so that the allergen is introduced under the skin surface. The skin is observed closely for signs of a reaction, usually swelling and redness of the site. Results are usually obtained within about 20 minutes, and several suspected allergens can be tested at the same time. In the case of respiratory tract symptoms provoked by pollen, house dust mite excreta or animal dander there is a 75% agreement between the results of provocation tests and skin tests. The concordance between skin tests and systems from ingested allergens is much lower indeed, skin–prick testing is only of limited help in the diagnosis of food allergic disease. Scratch testing is imprecise and unhelpful. Intracutaneous tests, in which allergens are injected more deeply into the skin, are reproducible and have an occasional role, but they are in general less specific, less well authenticated and more hazardous than prick tests.

Patch Tests

Patch tests are used principally to identify causative allergens in suspected allergic contact dermatitis. They should not be used in patients with acute eczema as results are misleading. The allergens are formulated in appropriate concentrations and placed in shallow aluminum wells of about 1 cm2 – Finn chambers. The wells are applied in strips to the patient’s back and kept in place by hypo–allergic tape. The skin is marked appropriately, and the patient is asked to keep the area dry. The patches are removed after 48hrs and the skin examined for a positive response, characterized by itching and erythematous, all of which may extend beyond the margin of the patch in strongly positive responses. This reaction represents a cell–mediated, delayed hypersensitivity (Type IV) response. Irritants may cause a rather similar response, but the reaction is commonly painful rather than itchy, and epidermal necrosis may occur. Patch testing is simple and clinically useful, but the results are not always easy to interpret. The concentration and presentation of the allergen is critical, and the distinction between an allergic and an irritant response is not always clear.

Standard contact allergen batteries have been developed in different areas of the world to include most common allergens in contact dermatitis. These include metallic ions, rubber accelerators and antioxidants, topical drugs and other sensitizing substances. The composition of these batteries will vary from place to place and from time to time.

Serum Immunoglobulins

Serum immunoglobulins levels may be measured. This is an essential part of the investigation of immunodeficiency. In atopic individuals, the IgE level is often elevated and the IgA level is occasionally depressed, but Type I allergic responses can occur with a normal serum IgE level. A consistent grossly elevated IgE level is often associated with intestinal parasitic infestation and should not be attributed to atopy alone without further investigation.

Rast Test

It is a laboratory test performed on blood (see venipuncture). It tests for the amount of specific IgE antibodies in the blood (which are present if there is a “True” allergic reaction). The test is expensive, does not provide immediate information and results are comparable with skin–prick testing. Other tests include immunoglobulin measurements (see serum globulin electrophoresis) and the blood cell differential and/or absolute eosinophil count (increased eosinophils can indicate the presence of allergy).

Food Allergy Tests

Food allergies are usually tested by using various “Elimination” diets in which the suspected foods are eliminated from the diet for several weeks and then gradually re–introduced one at a time, while the person is observed for signs of an allergic reaction. Because food allergies are often affected by what the person thinks they might be allergic to, a double–blind test may be advised. In this test, suspected foods and placebos (inert substances) are given in a disguised form. The person being tested and the health care provider are both unaware of whether the particular substance being tested in that session is a placebo or a food substance (a third party knows the identities of the substances and identifies them with some sort of code). This test requires several sessions if more than one substance is under investigation.

Other Tests

Challenge tests may be carried out in organs other than the skin. Thus, suspected allergens may be inhaled in bronchial or nasal challenge tests and their effects assessed by measurements of pulmonary or nasal airways resistance. It is often difficult to distinguish allergic and irritant responses when these methods are used, but they are sometimes helpful in diagnosing and assessing response therapy. Similarly, oral challenges with specific foods may be of help in the assessment of food allergic disease, usually following a period on a diet, which excludes the allergen. A number of other tests of organic function may, of course, be called for, depending upon the presenting clinical symptoms. End–organ biopsy may also be of help e.g. in many skin disorders in gluten–sensitive enteropathy and in some auto–immune disorders.

Biopsy specimens may be examined histologically and by immunofluoresence techniques. Indirect immunofluoresence, a fluorescein–labeled reagent specific for human Immunoglobulins is used to reveal their deposition in the tissues – most commonly the skin. Indirect immunofluoresence is used to reveal auto antibodies. Here, cryostat sections of tissue containing the relevant antigens are incubated with the patient’s serum. A fluorescein labeled reagent is then added, thus revealing the sites at which Immunoglobulins has become bound to the tissue section.