Skin allergy tests are crucial diagnostic tools used to identify specific substances that trigger allergic reactions in a patient's skin. These tests are designed to introduce a microscopic amount of an allergen to the skin through various means, observing the body's immune response. Identifying these allergens is vital for managing conditions like contact dermatitis and other hypersensitivities. The process involves careful application, observation,
and interpretation to provide accurate insights into a patient's allergic profile.
Methods for Detecting Immediate and Delayed Reactions
Several methods are employed in skin allergy testing, each suited for different types of allergic responses. For immediate reactions, common tests include the skin prick test, where a needle or pin containing a small amount of allergen pricks the skin, typically on the forearm. The skin scratch test involves a deep dermal scratch with a lancet, though it is less commonly used due to a higher risk of infection. The intradermic test injects a tiny quantity of allergen just under the dermis using a hypodermic syringe, often used for drug allergies like penicillin or bee venom. The skin scrape test, which is painless and carries no infection risk as it only affects the superficial layer of the epidermis, involves a superficial scrape with the back of a needle.For delayed reactions, the patch test is the primary method. This involves applying a patch containing various allergens to the skin, usually on the back. These patches contain substances like latex, medications, preservatives, hair dyes, fragrances, resins, and metals. The patch test is specifically used to detect allergic contact dermatitis and does not test for hives or food allergies. The chemicals included in standard patch test kits are responsible for approximately 85-90 percent of contact allergic eczema cases. To ensure the skin is reacting appropriately, all skin allergy tests also include control substances like histamine (which most people react to) and glycerin (which most people do not react to). If the skin does not react as expected to these controls, the results for other allergens may be considered falsely negative.
The Patch Test Process and Interpretation of Results
The application of patch tests typically takes about half an hour, though the overall appointment may be longer due to the need for an extensive patient history. Tiny quantities of 25 to approximately 150 different allergens, housed in individual square plastic or round aluminum chambers, are applied to the upper back and secured with special hypoallergenic adhesive tape. These patches must remain undisturbed for at least 48 hours. Vigorous exercise or stretching can disrupt the test. At the second appointment, usually 48 hours later, the patches are removed. The test sites are marked with an indelible pen, and a preliminary reading is conducted. A third appointment, typically 24-48 hours after patch removal (72-96 hours after application), is necessary for a final reading. In some cases, a reading at 7 days may be requested, particularly for special metal series tests.The dermatologist or allergist interprets the results based on the International Contact Dermatitis Research Group Criteria. Reactions are categorized as no reaction (0), doubtful reaction (?), weak positive (1+), strong positive (2+), extreme positive (3+), or irritant reaction (IR). Doubtful reactions show faint erythema, while weak positives present with palpable erythema, infiltration, and papules. Strong positives are more severe, including vesicles, and extreme positives are even more intense with coalescing vesicles. It's important to note that the patch test has a sensitivity ranging between 11-38%, meaning false negative reactions can occur. False positive reactions can also result from irritant responses. If a false negative is suspected, further testing like skin prick or intradermal testing might be recommended.
