What Allergens Are Tested in a Patch Test?

A standard patch test screens for roughly 30 to 80 substances known to cause allergic contact dermatitis, a delayed skin reaction triggered by direct contact with an offending material. These substances span metals, fragrances, preservatives, rubber chemicals, dyes, resins, and topical medications. The exact lineup depends on which baseline series your dermatologist uses and whether supplemental panels are added for your specific exposures, but the core goal is the same: identify which everyday chemicals are making your skin react.

How a Patch Test Differs From Other Allergy Tests

Patch testing evaluates a fundamentally different kind of allergic reaction than the skin prick tests or blood tests most people associate with allergy clinics. Prick tests and blood IgE tests look for immediate reactions, the kind that cause hives, swelling, or anaphylaxis within minutes. A patch test, by contrast, detects delayed hypersensitivity reactions that take one to several days to develop after skin contact with a substance.¹ That delayed timeline is why patch tests require multiple clinic visits: small amounts of potential allergens are applied to your back under adhesive chambers, left in place for about 48 hours, then read by a clinician who checks for redness, swelling, or tiny blisters at each test site.

Reactions are scored using a standardized system developed by the International Contact Dermatitis Research Group, which grades everything from a faint pink mark (doubtful reaction) up to a spreading, blistered response (strong positive).² Some clinicians also perform a later reading around day seven, because certain allergens only reveal themselves after a longer delay. In one study, about 13% of patients had new positive reactions that only appeared at the seven-day mark, including reactions to metals, preservatives, and fragrances that were completely negative at the standard 48-hour reading.³

The Baseline Series and What It Covers

Every major dermatology society maintains a “baseline series,” a curated list of the most common contact allergens in their region. In Europe, the European baseline series is the standard; in North America, the North American Contact Dermatitis Group (NACDG) maintains its own core series. These lists are periodically updated as new allergens emerge and older ones become less relevant. A typical baseline series includes allergens from several broad categories, and most clinics start here before deciding whether additional panels are needed.

Metals

Metals are consistently the top offenders in patch testing worldwide. Nickel sulfate is the single most common positive result, reacting in roughly 18 to 20% of tested patients depending on the study population.⁴⁵ Nickel is everywhere: jewelry, belt buckles, zippers, eyeglass frames, phone cases, and laptop surfaces. Cobalt chloride and potassium dichromate (a chromium compound) are the other two metals in the standard trio, with positive rates around 6 to 7% and 3 to 6.5% respectively.⁴ Cobalt turns up in pigments, vitamin B12 supplements, and some metal alloys, while chromium exposure comes from leather tanning agents, cement, and certain paints.

Fragrances

Fragrance allergy is tricky because thousands of individual scent chemicals exist, and testing each one is impractical. The baseline series addresses this by including “fragrance mixes,” standardized blends of the most allergenic scent ingredients. Fragrance mix I and fragrance mix II together capture a broad swath of fragrance sensitivity, but they are not perfect nets. In a Swedish study, about 6.5% of patients reacted to fragrance mix I and 3.2% to fragrance mix II, yet roughly a quarter to a third of patients who reacted to individual fragrance ingredients tested negative to the mixes themselves.⁶ The most frequently allergenic individual fragrance chemicals in that study were tree moss extract, cinnamal, and citral. This gap explains why some clinics test the individual fragrance ingredients alongside the mixes. Myroxylon pereirae resin, also called balsam of Peru, is another fragrance-related allergen in the baseline series; it contains a cocktail of natural aromatic compounds found in perfumes, flavored foods, and some topical medications, and it triggered reactions in about 5% of patients in one large European dataset.⁵

Preservatives

Preservatives are arguably the allergen category that has shifted the most over the past two decades. The big story has been the rise of methylisothiazolinone (MI) and its older relative methylchloroisothiazolinone/methylisothiazolinone (MCI/MI). These biocides were widely adopted in cosmetics, household cleaners, and paints after formaldehyde-releasing preservatives fell out of favor, and sensitization rates climbed steeply. In North American data spanning more than two decades, methylisothiazolinone triggered positive reactions in about 12% of tested patients, making it one of the most common allergens in the entire series.⁷ Formaldehyde and formaldehyde-releasing agents like quaternium-15 remain in the baseline series too, each producing positive reactions in roughly 7 to 8% of patients in the same dataset.⁷ Updates to the European baseline series have adjusted the concentrations of these preservative test preparations to keep pace with real-world exposure patterns.⁸

Rubber Accelerators

If you react to rubber gloves, elastic waistbands, shoe soles, or certain adhesive bandages, rubber accelerator chemicals are usually the culprits. The baseline series includes thiuram mix, mercapto mix, and mercaptobenzothiazole to cover the main chemical families used to speed up rubber curing. Thiuram mix has the highest reactivity rate among these, with a pooled prevalence of about 2.5% across large patient populations, while mercapto mix and mercaptobenzothiazole each trigger reactions in under 1% of patients.⁹ These rates sound low, but the clinical relevance is high: more than half of people who test positive to thiuram mix, and roughly two-thirds of those positive to mercapto compounds, have dermatitis that is actually caused by that allergy rather than being a coincidental finding.⁹

Other Baseline Allergens

The baseline series also includes p-phenylenediamine (PPD), the main allergen in permanent hair dye and dark henna tattoos, which reacted in about 4% of a large Turkish cohort.⁵ Colophony (rosin), a natural resin found in adhesives, plasters, and stringed instrument rosin, is another standard inclusion. Epoxy resin shows up in industrial adhesives and coatings and is tested in the baseline series as well. Topical antibiotics like neomycin sulfate have long been included, though positive reaction rates have been declining over time as usage patterns change. Wool alcohols (lanolin), found in moisturizers and ointments, round out the list for people who react to topical creams that are supposed to help their skin.

Supplemental and Specialized Panels

When the baseline series alone does not explain a patient’s dermatitis, clinicians can add targeted panels. These supplemental series are designed for specific occupational exposures or anatomical patterns of rash.

A cosmetics and personal care series adds individual fragrance chemicals, surfactants like cocamidopropyl betaine (common in shampoos), and additional preservatives. People with eczema are particularly worth watching here: atopic dermatitis has been linked to higher rates of reactions to cocamidopropyl betaine, wool alcohol, lanolin, and certain other topical ingredients.¹⁰

A dental materials series tests for allergens found in fillings, crowns, dentures, and orthodontic appliances. This panel covers materials like eugenol, various acrylates, and metals such as palladium and gold that appear in dental alloys. Clinicians turn to this panel when patients develop mouth sores, burning sensations, or facial rashes that seem linked to dental work.¹¹

Other available panels include textile dye series (disperse dyes used in synthetic fabrics), hairdressing series (additional hair dye chemicals and bleaching agents), plant series (sesquiterpene lactones, Compositae extracts), and medicament series (topical steroids, local anesthetics, and additional antibiotics). The choice of panels is driven by the patient’s exposure history and where the rash appears on their body.

Medical Devices as an Emerging Allergen Source

One of the more surprising developments in recent years has been allergic contact dermatitis caused by wearable medical devices, particularly continuous glucose monitoring sensors and insulin pumps used by people with diabetes. The adhesives in these devices often contain acrylate compounds that are not part of the standard baseline series. Isobornyl acrylate, in particular, sensitized a large number of patients wearing one widely used glucose sensor system and became a well-documented contact allergen in both children and adults.¹²¹³

This situation highlights a recurring challenge: manufacturers reformulate products, introduce new chemicals, and the standard patch test series can lag behind. When a patient reacts to a specific product but tests negative to everything in the baseline and supplemental panels, the dermatologist may need to test the patient’s own products or request the device manufacturer’s ingredient list to identify the offending chemical. The glucose sensor story also shows that even “medical-grade” adhesives are not automatically hypoallergenic, a misconception worth keeping in mind if you develop a persistent rash under any adhesive device.

Photopatch Testing

Standard patch testing will miss one important subtype of contact allergy: reactions that only happen when the skin is simultaneously exposed to a chemical and ultraviolet light. Sunscreen ingredients and certain anti-inflammatory drugs are the classic triggers. To catch these, clinicians use photopatch testing, which applies duplicate sets of allergens to the back. After the standard occlusion period, one set is exposed to a controlled dose of UVA light while the other remains covered.¹⁴ A reaction that appears only on the irradiated side confirms a photoallergic response, as opposed to a standard contact allergy that would show up on both sides.

The European photopatch test series includes UV filters found in sunscreens (like benzophenone-3 and octocrylene), topical nonsteroidal anti-inflammatory drugs (like ketoprofen), and antimicrobial agents. A large European multicenter study followed this consensus approach to identify sunscreen photoallergens across multiple countries.¹⁵ Photoallergic contact dermatitis is far less common than ordinary contact dermatitis, but it is worth considering if your rash consistently appears on sun-exposed areas and worsens with sunscreen use rather than improving.¹⁶

Patch Testing in Children

Allergic contact dermatitis in children is more common than many clinicians historically assumed, and it is increasingly recognized as a contributor to stubborn childhood eczema. The allergens that turn up most often in pediatric patch testing overlap substantially with the adult list but with a few differences in ranking. In children, the most common allergens are nickel, thimerosal, cobalt, fragrances, lanolin, and neomycin. In adolescents, the pattern shifts slightly: nickel still leads, followed by thimerosal, cobalt, fragrance, potassium dichromate, and Myroxylon pereirae.¹⁷

Pediatric patch testing uses the same allergen series as adult testing, though some clinics adjust concentrations for very young children or reduce the number of allergens tested. One complication is that many children referred for patch testing already carry an atopic dermatitis diagnosis, and distinguishing a new contact allergy from a flare of existing eczema requires clinical judgment. Nonetheless, testing is encouraged when a child’s dermatitis persists despite standard treatment, since identifying and avoiding a contact allergen can dramatically improve symptoms.¹⁸

Why Allergen Trends Shift Over Time

The list of allergens tested in a patch test is not static. As consumer products evolve, new sensitizers emerge and old ones decline. European data comparing patch test results across a roughly 15-year span found that reactions to MCI/MI, Myroxylon pereirae, PPD, and fragrance mix I all increased, while reactions to neomycin and certain older preservatives like clioquinol and primin dropped significantly.⁵ These shifts reflect real changes in what people are exposed to. Neomycin is used less often in over-the-counter antibiotic ointments than it once was, so fewer people become sensitized. Meanwhile, the boom in “paraben-free” cosmetics led manufacturers to substitute isothiazolinone preservatives, which turned out to be even more allergenic, producing the spike in MI sensitization described earlier.

Regulatory action can also reshape allergen trends. The European Union restricted MI in leave-on cosmetics in 2016 after the surge in contact allergy became clear, and early signals suggest sensitization rates may have started to plateau. Fragrance regulations requiring ingredient labeling have made it somewhat easier for sensitized individuals to avoid their triggers, though vague terms like “parfum” on labels can still obscure specific allergens.

What Can Go Wrong During Testing

Patch testing is generally safe and well tolerated, but a few things can compromise the results. One recognized phenomenon is “excited skin syndrome,” sometimes called “angry back,” where a strong positive reaction to one allergen causes the surrounding skin to become hyper-reactive, producing false-positive responses to nearby test sites. This has been documented even in patients taking immunosuppressive medications that might be expected to dampen allergic responses.¹⁹ When a clinician suspects angry back, the questionable positives are usually retested individually at a later date to confirm or rule them out.

Oral corticosteroids and other systemic immunosuppressants can suppress patch test reactions and lead to false negatives. Most dermatologists ask patients to stop systemic steroids before testing when it is safe to do so. Topical steroids on the back are also avoided in the days before application. Sunburned or heavily tanned skin on the back can interfere with readings, which is why testing is often scheduled outside of peak summer months.

What Happens After You Get Your Results

Identifying your allergens is only useful if you can actually avoid them, and this is where the clinical payoff of patch testing becomes tangible. In a retrospective analysis of patients who underwent patch testing followed by allergen avoidance counseling, 81% reported improvement in their dermatitis afterward.²⁰ That is a high success rate for a condition that often frustrates patients for years before the right trigger is identified.

Avoidance counseling typically involves reviewing your positive allergens, identifying the products and materials that contain them, and finding safe substitutes. Databases like the Contact Allergen Management Program (CAMP) and the Contact Allergen Replacement Database (CARD) allow clinicians to generate personalized “safe product” lists based on your specific allergen profile. For metals like nickel, practical strategies include using a dimethylglyoxime spot test on jewelry and metal objects to check for nickel release before prolonged skin contact. For preservatives and fragrances, the focus shifts to reading ingredient labels carefully, a skill that takes practice given how many different chemical names a single allergen can hide behind.

One nuance worth understanding: a positive patch test means your immune system is sensitized to that substance, but it does not automatically mean that substance is causing your current rash. Clinical relevance has to be determined by matching the positive allergen to your actual exposure pattern. A positive reaction to nickel in someone who never wears metal jewelry and has a rash on their eyelids is probably not the explanation, whereas a positive to a preservative found in their daily moisturizer and eye cream almost certainly is. This distinction between sensitization and clinical relevance is something your dermatologist evaluates for each positive result.

Testing Your Own Products

Sometimes the standard panels and supplemental series still do not catch the allergen responsible for a patient’s dermatitis. In these cases, dermatologists can apply a patient’s own products directly to the skin using the same patch test technique. Cosmetics, moisturizers, workplace chemicals, and household cleaners can all be tested this way, though the clinic needs to confirm the product is safe to apply in occluded form (some industrial chemicals are too irritating or corrosive for direct skin application). This approach is particularly useful when a product contains a proprietary ingredient blend that is not represented in any commercial test series. The limitation is that even if a product causes a positive reaction, you may not know which specific ingredient within it is to blame without further testing with individual components.

1
PubMed Central. Prick, patch or blood test? A simple guide to allergy testing
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PubMed. Allergy patch test reading from photographic images: disagreement on ICDRG grading but agreement on simplified tripartite reading
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PubMed. Patch test results of the European baseline series among 1309 patients in Turkey between 2013 and 2019
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PubMed Central. Simultaneous patch testing with fragrance mix I, fragrance mix II and their ingredients in southern Sweden between 2009 and 2015
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PubMed. Continuous glucose monitoring systems give contact dermatitis in children and adults despite efforts of providing less ‘allergy- prone’ devices
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PubMed Central. Patient Reported Improvement After Patch Testing and Allergen Avoidance Counseling: A Retrospective Analysis