My beauty editor friends are honest. Want to know if the viral Rose Skin Co. laser-hair remover really works, or if that trending ice-water facial is B.S.? They’ll give it to you straight. Need to know if your recent curtain bangs were a good idea? Get ready for some potentially painful texts. So I expected nothing less than the whole truth when I recently polled my editor friends to find out if anyone is actually patch-testing their new products before slathering them on. And to my surprise, I was met with crickets.
If you’re unfamiliar, at-home patch testing involves dabbing a teeny amount of product to an inconspicuous area (typically behind the ear) to rule out contact dermatitis and allergic reactions. But despite being loudly preached by experts—especially by beauty editors—most of my friends admitted to never patch-testing their products. And honestly, I get it: Patch testing might feel like an annoying science experiment you’d rather avoid, but it’s actually invaluable for providing a specific answer to which ingredients are irritating your skin.
So ahead, I’ve compiled a comprehensive guide to demystify the patch-testing process so you’ll have all the information you need to get to the bottom of your skin allergies and sensitivities. To help, I chatted with two board-certified dermatologists (Kristina Collins, MD, and Karan Lal, DO); a board-certified allergist, Martin Smith, MD; and a cosmetic chemist, Ginger King, about what a patch test is, why you should patch-test your skin-care products at home, and what happens during a professional patch test. Let’s dive in.
Experts in this article
- Martin Smith, MD, a double board-certified allergist and immunologist in Cleveland, OH
- Kristina Collins, MD, a double board-certified dermatologist based in Georgetown, TX
- Karan Lal, DO, a double-board-certified dermatologist in Scottsdale, AZ
- Ginger King, a cosmetic chemist and product developer in New Jersey
What is a patch test?
A patch test is a non-invasive skin test to identify ingredients you’re allergic or sensitive to. There are two versions: the DIY version and the professional version. Often seen on the back of a new hair dye, a DIY patch test simply refers to dabbing a bit of product on a small area (typically behind your ear or inside of your forearm) to see how your skin reacts after 24 or more hours.
A professional patch test, however, can be done with a board-certified dermatologist or allergist, and the process starts in the office but extends beyond. “You keep the allergen patches they apply on, but you return to the office at 48 hours, 72 hours, and 96 hours,” Dr. Lal says. “The patches come off at those visits, and we take pictures and assess the response to the allergens.” According to Dr. Lal, early reactions are usually indicative of irritant reactions, whereas allergens that appear at 72 hours and 96 hours are due to skin allergens.
While the simplified at-home version saves time and money, it’s considered “less scientific,” according to Dr. Lal. Nonetheless, dermatologists, experts, and beauty editors often recommend this method, especially when introducing new skin-care products. However, the jury’s still out on how many people actually follow through. To illustrate this, I also contacted my friend with sensitive skin, who confessed to never having patch-tested her products. (Even Dr. Lal admitted that he doesn’t think everyone’s doing it: “I can’t imagine trying to test everything you ever use,” he says. “It could get overwhelming.”)
Why you should patch-test your skin care
Patch-testing your skin-care products at home can help you catch reactions early, saving time, stress, and headaches in the long run. While professional patch testing is the way to go for comprehensive results, Dr. Collins explains that there are situations where conducting patch tests for skin-care products at home may suffice, especially when a skin reaction hasn’t been a recurring issue.
“Say you just tried two new products, your skin got really itchy, and you’re trying to figure out which one of the two things did it,” says Dr. Collins. “That could be a good time to do a patch test at home.” In this case, you’d be able to patch-test both products, wait 24 hours, and stop using the formula that causes a reaction.
You should also, technically, patch test new products before incorporating them into your skin-care routine, especially if they contain active ingredients you aren’t already using. This step is essential to rule out any potential issues and prevent them from occurring. Instead of immediately slathering your face in that new moisturizer packed with acid and fragrance, consider taking a day to test it behind your ear. This simple precaution can help you feel more confident about your products and their effects on your face.
What is contact dermatitis?
“Contact dermatitis is a delayed-type allergic skin reaction that happens from applying products directly to your skin,” Dr. Martin says. It differs from an immediate allergy you can experience from foods, seasonal pollen, or insect stings. Usually, a reaction happens three to five days after you use an offending product, and you can have a constant rash even if you use that product once in three weeks.
“The two main types of contact dermatitis are irritant and allergic,” says Dr. Martin. Even though both irritant and allergic dermatitis are classified as contact dermatitis, they’re different. Dr. Lal says allergic dermatitis kicks your immune system into gear at a molecular level. On the other hand, Dr. Collins explains that sensitivity can make your skin burn or sting right away: “Just like some people get really irritated and their eyes tear up when peeling onions, some people are bothered similarly with ingredients.”
Contact dermatitis has the potential to occur anywhere on the body. Dr. Collins notes that some of the most common areas where patients experience it include their eyelids, lips, face, and hands. She explains that while you may associate the location of your dermatitis with a product you use in that area, such as an eye cream on your eyelids, this may not always be the case.
“Sometimes, reactions can be a little deceiving,” says Dr. Collins. “One of the most common reasons people undergo patch testing is because of eyelid dermatitis, which is a frequent manifestation of contact dermatitis due to the thinness of the eyelid skin.” Basically, because of its thinness, your eyelids are more reactive than other parts of your body. So you may think the culprit is a new eye cream, but it could also be something like your nail polish. “When you scratch your face, the delicate eyelid skin becomes susceptible to developing an allergic reaction,” Dr. Collins says.
Her suggestion? If you’re attempting to identify the cause of a reaction, “start by keeping a diary and logging everything that comes into contact with your skin,” says Dr. Collins, “whether it’s shampoo, conditioner, moisturizer, cosmetics, or anything else.” This can be helpful in determining which products to patch-test at home—or, consequently, helpful in stressing you out enough that you go for a professional test with your dermatologist.
Common allergens in skin care and cosmetic products
Whether you have a reaction to a product at home or to the allergen stickers in a doctor’s office, the underlying irritant could be the same allergen. If you’re not sure where to start and you don’t have access to a dermatologist for professional patch testing (it can be expensive without insurance), you may want to avoid the following common allergens:
Fragrance
“The most common allergens are fragrances, including botanical compounds such as essential oils,” Dr. Smith says. Fragrance allergies are so common among Dr. Collins’ patients that she encourages people to avoid them entirely.
Lanolin
Lanolin is an ingredient from sheep’s wool. It’s often used in skin-care products for its moisturizing benefits, though our experts point out that it’s also a common allergen. “The scary thing is that many brands marketed to babies and people with eczema or sensitive skin contain allergens such as lanolin,” says Dr. Smith.
Formaldehyde-releasing preservatives
Quaternium-15 is a super-common formaldehyde-releasing preservative used to prolong the shelf-life of products. It’s also strongly disliked by all of our interviewed experts due to its high risk of irritation. According to a 2022 Pubmed clinical review, it’s a known allergen, which will typically be listed as one of the final ingredients on a product’s ingredients list.
Nickel
Nickel isn’t just limited to the buttons of your jeans or zippers—it’s also found in certain cosmetics, like mascara. According to Dr. Smith nickel can be found in trace amounts in colorants, particularly in blue and green pigments, but it’s usually only cause for concern if there’s a reaction. Reactions to most of these allergens may appear as contact dermatitis.
How to patch-test skin-care products at home
If you’re going with the at-home method, Dr. Smith recommends applying a quarter-size amount of the product you want to test to the inside of your forearm or somewhere else you won’t accidentally rub the product off. For facial products, he says you can also consider testing behind your ear.
Dr. Smith says you’ll want to apply the product twice daily for at least a few hours. But if you don’t want to reapply twice daily, he suggests a different option: You can apply it just once and then “cover the spot with a bandage and peel it off in four days” (or sooner, if you feel irritation). This time is necessary because, as we’ve discussed, contact dermatitis is a delayed-type allergic reaction that could take days to develop into a rash.
However, Dr. Smith says you should react quickly if you see or feel a reaction. “Immediately wash the area with a gentle fragrance-free soap,” he says. “If the rash worsens, which often happens, you can try an OTC cortisone cream, or better yet, go in and see your doctor, allergist, or dermatologist.”
Symptoms to look out for during an at-home patch test
According to Dr. Smith, symptoms to look out for during an at-home patch test include signs of skin inflammation where you apply the product, such as redness, swelling, raised bumps, blisters, or peeling. But, chances are, you might feel a response like intense itching, which he also says is a pretty common symptom.
If your symptoms don’t improve within a few minutes or hours (or if they worsen, despite rinsing and applying cortisone cream), you should see a dermatologist or doctor who can possibly provide stronger topical or oral meds to ease your symptoms.
What happens during a professional patch test?
During a professional patch test, your provider will test your skin for common allergies, but let’s dive deeper. For starters, Dr. Collins explains two types of patch tests: the True Test and the North American 80 Comprehensive Series test. The former is “pretty limited” (she calls it a “baby test”), but it’s the one most often used in dermatology and allergy offices. The latter is a more comprehensive option that tests for 80 allergens compared to the True Test’s 35.
Once you and your healthcare provider have chosen the appropriate test, the procedural steps follow. Your provider will prepare and cleanse your back, and then the main phase begins: creating a grid-like pattern on your back and sticking individual patches (each containing a different allergen) onto your skin. There is the “rare chance” your skin reacts immediately. In that case, Dr. Lal tells us your provider will remove the patches, stop treatment, and develop a new game plan. Otherwise, your provider will send you on your way, and the waiting game will begin.
Typically, you’ll be asked to return for one or more follow-ups that week. Dr. Collins explains that if you visit on a Monday, your provider will likely schedule your return for Wednesday to remove the patches and then on Friday for a “final look.” During this final examination, they can observe any reactions that occurred on Wednesday, as well as those that developed by Friday, grading each allergen accordingly before giving you a list of your final allergens (if any).
To add another layer of complexity, in rare cases, “there could be an ingredient that’s just really specific to an individual that’s not on either of the patch testing series and in that case, your provider can design a specific test for that person,” Dr. Collins says. She explains that some ingredients are occupation-specific, such as those encountered by nail artists and hairstylists, which the general public may interact with less frequently. Consequently, these ingredients are not typically included in standard patch tests.
Final takeaway
If you want to avoid a trip to the derm, patch-testing your skin care at home is a good first step to ruling out reactions or (some) allergens. However, if your skin is irritated, reactive, and doesn’t seem to be improving—or you want to leave the rash-sleuthing to the professionals—professional allergen testing is a surefire way to pinpoint whether you’re dealing with an irritant or allergic contact dermatitis. After all, dermatologists and allergists spend a good chunk of their lives studying to help you, so don’t be afraid to utilize their very smart brains for help.
Hero Illustration by Janet Mac