Scratch test

Skin allergy testing is a method for medical diagnosis of allergies that attempts to provoke a small, controlled, allergic response.

A microscopic amount of an allergen is introduced to a patient's skin by various means:

If an immuno-response is seen in the form of a rash, urticaria (hives), or (worse) anaphylaxis it can be concluded that the patient has a hypersensitivity (or allergy) to that allergen. Further testing can be done to identify the particular allergen.

The "skin scratch test" as it is called, is not very commonly used due to increased likelihood of infection. On the other hand, the "skin scrape test" is painless, does not leave residual pigmentation and does not have a risk of infection, since it is limited to the superficial layer of the skin.

Some allergies are identified in a few minutes but others may take several days. In all cases where the test is positive, the skin will become raised, red and appear itchy. The results are recorded - larger wheals indicating that the subject is more sensitive to that particular allergen. A negative test does not conclusively rule out an allergy; occasionally, the concentration needs to be adjusted, or the body fails to elicit a response.

In the prick, scratch and scrape tests, a few drops of the purified allergen are gently pricked on to the skin surface, usually the forearm. This test is usually done in order to identify allergies to pet dander, dust, pollen, foods or dust mites. Intradermal injections are done by injecting a small amount of allergen just beneath the skin surface. The test is done to assess allergies to drugs like penicillin or bee venom.

To ensure that the skin is reacting in the way it is supposed to, all skin allergy tests are also performed with proven allergens like histamine or glycerin. The majority of people do react to histamine and do not react to glycerin. If the skin does not react appropriately to these allergens then it most likely will not react to the other allergens. These results are interpreted as falsely negative.

...
Wikipedia