A patch test identifies specific substances causing allergic contact dermatitis, a delayed skin reaction. It pinpoints triggers like rashes or eczema, guiding individuals toward effective allergen avoidance. Medical professionals determine which allergens cause a patient’s skin condition by reproducing a localized allergic reaction.
Common Categories of Allergens Tested
Metals are a common group, with nickel being the most prevalent allergen, found in jewelry, belt buckles, and tools. Cobalt and chromium are also commonly tested metals, present in dyes, cements, and leather products.
Fragrances are detected through specific fragrance mixes and balsam of Peru. These are common ingredients in cosmetics, personal care products, and detergents, and balsam of Peru alone can identify approximately 50% of fragrance allergies. Preservatives are frequently implicated in skin reactions, including formaldehyde-releasing agents like quaternium-15 and DMDM hydantoin, as well as methylisothiazolinone (MI) and methylchloroisothiazolinone (MCI). These substances are found in shampoos, lotions, and some industrial products.
Rubber chemicals like thiram and mercaptobenzothiazole (MBT) are tested, found in gloves, elastic bands, and footwear. Topical medications, including antibiotics like neomycin and bacitracin, can cause reactions. Paraphenylenediamine (PPD), used in permanent hair dyes and black henna tattoos, often causes hair dye allergies. Other tested allergens include epoxy resins (adhesives) and acrylates (nail products, dental materials).
Specialized and Expanded Allergen Panels
Beyond standard allergens, dermatologists use specialized panels for less typical exposures. These panels investigate allergens relevant to a patient’s occupation, hobbies, or lifestyle, identifying triggers routine tests might miss.
For individuals with suspected reactions to dental materials, a dental series panel is available, which includes metals like nickel, cobalt, and gold, along with acrylics and methacrylates such as methyl methacrylate and 2-hydroxyethyl methacrylate (HEMA). Professionals like hairdressers, exposed to many chemicals, may undergo testing with a hairdresser series that encompasses substances beyond common hair dye components. Bakers, exposed to various flours and additives, might be tested with a bakers series.
Panels for textile dyes are used when clothing or fabrics are suspected causes of dermatitis, while photographic chemicals can be tested for individuals in that industry. These specialized panels are useful when initial patch testing does not yield clear answers or when specific occupational or recreational exposures are highly suspected. They allow for a more targeted investigation into potential contact allergens.
How Allergens are Applied and Evaluated
The process of a patch test involves the application of small, standardized quantities of allergens onto the skin. These allergens are contained within small discs or chambers affixed to adhesive patches, placed on the patient’s back. A single test can involve 30 to over a hundred different allergens.
The patches remain on the skin for approximately 48 hours for a potential allergic reaction to develop. During this period, patients are advised to keep the area dry and avoid activities that might dislodge the patches. After 48 hours, the patches are removed, and the skin is initially assessed.
A follow-up reading occurs 72 to 96 hours after the initial application, or up to seven days, as allergic contact dermatitis reactions are delayed. A positive reaction appears as redness, swelling, small bumps (papules), or blisters (vesicles) at the site where the allergen was applied. These reactions are then graded by the dermatologist to determine the strength of the allergy.
Investigating Specific or Uncommon Allergens
When standard or specialized patch testing does not identify a clear cause, or if a patient suspects a specific product not typically included in commercial panels, dermatologists can pursue further investigation. This involves “user-applied” patch testing, where a small amount of the patient’s own suspected product is applied to the skin. This could include cosmetics, shampoos, lotions, or household cleaners that the individual regularly uses.
A thorough patient history is important in guiding the investigation of unusual or highly specific allergens. The dermatologist gathers detailed information about the patient’s daily routine, hobbies, occupation, and any products they use, which helps narrow down potential culprits. Patients may be asked to bring product packaging or safety data sheets to their appointment, providing valuable details about ingredients for testing. This personalized approach helps ensure that even less common or novel allergens are considered when trying to identify the source of a patient’s skin reaction.