An allergic child or adult should have a complete physical examination, history, and laboratory tests, such as blood count, urinalysis, and nasal smear. Following this there are three types of skin tests which can be performed to determine the allergies.
The most frequently used is the scratch test in which a small amount of specific substance is introduced onto the skin and a small scratch made, usually on the forearm. If a small wheal (which looks like a small mosquito bite) develops, this is a suspicious, positive reaction to the allergen in question. Some patients may show a positive skin-test reaction to substances which they have never eaten or been exposed to. Such a reaction does not necessarily indicate that they are allergic to these substances.
An intradermal test is one in which the antigen is injected into the dermal (deeper) layer of the skin with a tiny needle. This test is more sensitive and may give a strongly positive reaction if the person is allergic to the material. A patch test, in which the allergic material is placed on the skin as a sort of patch and left for twenty-four to forty-eight hours, is another method of testing for sensitivity.
Passive transfer is a special method which is used when the skin cannot be involved in testing, as in severe eczema, or with children who are too difficult to test. It is also used to spare the individual the test procedures. The serum obtained from the allergic person is injected into the skin of a number of areas of a normal person. A few days later the allergic extracts are introduced into the same areas on the skin and the tests are read.
Allergy can also be tested by placing a drop of antigen into the eye of a sensitive person. The reaction is noted by comparing it with the response to a normal control.