The most important test in the assessment of eczema is the patch test. This test is usually performed at the hospital dermatology clinic but increasing numbers of GPs with a special interest in allergy are also able to provide this service.
In allergic eczema, the offending allergen or antigen generates an antibody response. Repeated exposure over time to the antigen results in increased sensitisation of the immune system and the generation of an allergic reaction within the skin. The patch test enables the doctor to establish which agents the person has been sensitised to. Very often it is established that the person is sensitised to several different agents and has a form of multiple allergy. The theory behind this approach is that if the allergen can be identified and avoided the allergic reaction in the skin can be prevented, thus achieving prevention of relapses of the eczema.
Patch testing In patch testing 30 to 90 patches containing common allergens are applied to the skin, usually on the back. After 96 hours the patches are removed. If the person is allergic to one of the test substances there will usually be visible signs on the skin of an allergic reaction.
Another commonly used test is called the RAST (radio-allergo sorbent test). This test is performed on a sample of blood but some doctors question the value of the test because the information provided is non-specific. The test simply indicates that a hypersensitivity state exists but does not pinpoint the relationship between a particular allergen and the hypersensitivity reaction.
Skin prick test This test is often confused with the skin prick test, which is carried out on the forearm. A small drop of allergen (pollen, dust etc) is introduced with a needle into the outermost layer of the skin. If the person is sensitive to the particular allergen a weal appears.
Many health professionals question the value of the two preceding tests especially in relation to food allergy. The elimination diet approach, which takes some time, is generally regarded as the most reliable means of identifyinga food allergy.
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