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).