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A patch test is a diagnostic method used to determine which specific substances cause Allergy inflammation of a patient's skin.
Patch testing helps identify which substances may be causing a delayed-type allergic reaction in a patient and may identify allergens not identified by blood testing or skin prick testing. It is intended to produce a local allergic reaction on a small area of the patient's back, where the diluted chemicals were planted.
The chemicals included in the patch test kit are the offenders in approximately 85–90 percent of contact allergic eczema and include chemicals present in metals ( e.g., nickel), rubber, leather, formaldehyde, lanolin, fragrance, toiletries, hair dyes, medicine, pharmaceutical items, food, drink, preservative, and other additives.
The first step in becoming allergic is sensitization. When skin is exposed to an allergen, the antigen-presenting cells (APCs) – also known as Langerhans cell or Dermal Dendritic Cell – phagocyte the substance, break it down to smaller components and present them on their surface bound major histocompatibility complex type two (MHC-II) molecules. The APC then travels to a lymph node, where it presents the displayed allergen to a CD4+ T-cell, or T-helper cell. The T-cell undergoes clonal expansion and some clones of the newly formed antigen specific sensitized T-cells travel back to the site of antigen exposure.
When the skin is again exposed to the antigen, the memory t-cells in the skin recognize the antigen and produce cytokines (chemical signals), which cause more T-cells to migrate from . This starts a complex immune cascade leading to skin inflammation, itching, and the typical rash of contact dermatitis. In general, it takes 2–4 days for a response in patch testing to develop. The patch test is just induction of contact dermatitis in a small area.
Patch Testing for cosmetic and skincare products can be broken down into a variety of different categories, including the following:
The patch test has a poor sensitivity ranging between 11-38%, meaning that false negative reactions are common with the patch test. False positive reactions can also occur as a result of irritant reactions. If the patch test yields a false negative result, then skin prick or intradermal testing may be recommended.
The most frequent allergen recorded in many research studies around the world is nickel. Nickel allergy is more prevalent in young women and is especially associated with ear piercing or any nickel-containing watch, belt, zipper, or jewelry. Other common allergens are surveyed in North America by the North American Contact Dermatitis Group (NACDG).
Certain and flavorings can cause allergic reactions around and in the mouth, around the anus and vulva as food allergens pass out of the body, or cause a widespread rash on the skin. Allergens such as nickel, balsam of Peru, parabens, sodium benzoate, or cinnamaldehyde may worsen or cause skin rashes.
Foods that cause urticaria (hives) or anaphylaxis (such as peanuts) cause a type I hypersensitivity reaction whereby the part of the food molecule is directly recognized by cells close to the skin, called mast cells. Mast cells have antibodies on their surface called immunoglobulin E (IgE). These act as receptors, and if they recognize the allergen, they release their contents, causing an immediate allergic reaction. Type I reactions like anaphylaxis are immediate and do not take 2 to 4 days to appear.
In a study of patients with chronic hives who were patch tested, those who were found allergic and avoided all contact with their allergen, including dietary intake, stopped having hives. Those who started eating their allergen again had recurrence of their hives.Guerra L, Rogkakou A, Massacane P, Gamalero C, Compalati E, Zanella C, Scordamaglia A, Canonica WG, Passalacqua G. Role of contact sensitization in chronic urticaria. J Am Acad Dermatol 2007; 56:88–90.
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