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:
- Skin prick test: pricking the skin with a needle or pin containing a small amount of the allergen.
- Skin scratch test: a deep dermic scratch is performed with help of the blunt bottom of a lancet.
- Intradermic test: a tiny quantity of allergen is injected under the dermis with a hypodermic syringe.
- Skin scrape Test: a superficial scrape is performed with help of the bevel of a needle to remove the superficial layer of the epidermis.
- Patch test: applying a patch to the skin, where the patch contains the allergen
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.