You may have read (and possibly ignored) a suggestion on a cosmetic product you own to “patch test before use.” Or perhaps you’re having an allergic reaction to some product in your home, but can’t figure out what the culprit is.

Either way, patch testing is widely suggested as both prevention and solution for deciphering allergies, especially skin allergies.

Many of us, though, have no clue exactly what it is or how to do it. You’re in luck. We’ll tell you everything you need to know about a patch test.

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Illustration by Maya Chastain

There are two types of patch testing. You can patch test at-home to test out a cosmetic or skin care product before you fully incorporate it into your routine, to determine if your skin likes the product.

If it doesn’t, you’ll see irritation, redness, or itching, which could also indicate an allergic reaction. That said, an at-home patch test cannot tell you if a specific ingredient is causing a reaction. It can only tell you that your skin doesn’t like a product.

The second and more determinate type of patch testing is done at a doctor’s office. This is set up to uncover which ingredients you might be allergic to.

Dermatologist Dr. Angelo Landriscina explains that, “It specifically tests for allergic contact dermatitis, which is a type of allergy where a person develops a rash at the point of contact where an allergen touches the skin.

“It can also uncover a phenomenon called contact urticaria, in which patients develop localized hives quickly after coming into contact with an allergen.”

An in-office patch test will test for most common allergens, such as:

  • metals (nickel and cobalt)
  • fragrance (Fragrance Mix I, Fragrance Mix II)
  • Balsam of Peru resin (harvested from trees grown in Central America for its spicy vanilla-like fragrance)
  • formaldehyde
  • topical antibiotics (Neomycin and Bacitracin)
  • para-phenylenediamine (PPD), found in black hair dyes
  • rubber
  • leather
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Dana Murray, a licensed esthetician, tells us that those with sensitive skin especially should watch for an immediate reaction after application of a product when patch testing at home. Otherwise, check in on that area for the next 24 to 72 hours for any adverse effects.

Landriscina notes the medical patch testing process usually takes 5 days, with one exception.

“Your doctor may ask you to do a repeat open application test,” he says, of some cases. “This is a type of test used when there is a specific product that is suspected to be causing the contact dermatitis in question. In this test, the patient applies the suspected product to an area where there is no rash… for 1 to 2 weeks to see if a reaction occurs.”

Medical patch testing does look quite different from the DIY version. If you’re not clear on how to assess your skin condition, take the party to a doctor’s office.

Before the patch test, a dermatologist will be able to properly assess the state of your skin. You might want to even bring photos so they can get a 24/7 look.

“On the first day of testing, the patches will be affixed to your skin, usually on the back,” says Landriscina. These patches contain potential allergens, and can sometimes include your specific products.

Then get ready for visitation because you will return to the doctor within 48 hours and usually 96 hours later for readings. Readings can be determined as:

  • negative
  • uncertain
  • irritation
  • weak positive
  • strong positive
  • extreme

“Reactions to the allergens in the patch test are graded on a scale that takes into account what kind of rash develops at the site,” explains Landriscina. “This can range from faint redness to the formation of blisters.”

An advantage of this type of patch testing is that you can be tested for many potential allergens at once.

“Many dermatologists use a prepackaged patch test called a TRUE test which tests for 35 common allergens,” Landriscina says.

“Select dermatologists who specialize in contact dermatitis will carry out a more extensive and customizable test using the Finn chamber technique — this technique allows your doctor to tailor the test to certain types of allergens that they might suspect, and they can even test your own products.”

Unlike an at-home test, a medical test leaves the brainpower of figuring out what to test, assessing your skin, and interpreting the results to your doctor. However, you will still need to be mindful of a few things while you complete this process:

  • Don’t get the area wet as this could cause allergens to run and make the test unreadable.
  • If you have to shower, opt for a sponge bath.
  • Avoid exercise during the test.
  • Do not expose patches to the sun.
  • Avoid skin products on these areas.

“Certain medications might also confound the results,” says Landriscina. “Your doctor will review your medications before starting the test.”

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Illustration by Maya Chastain

If you’re worried about an allergic reaction, you probably want to avoid applying the product on a publicly visible part of your skin, like your cheek or back of your hand. This isn’t the time for swatching, folks!

For at-home patch tests, Murray notes that many people incorrectly patch test in completely different areas, like their legs, than where the product is intended to work.

“The skin has different thickness all over the body and may result in different reactions in different areas,” she says, explaining how unproductive this can be.

So where does that leave us? Well, start by taking a dime-sized amount of product. And then, as Murray suggests, start “at your jawline, on your neck directly under your ear for facial products. It’s the closest facial skin and could easily be hidden with hair.”

This is the same for hair dye as well. The back of your head or your neck, where your hair can cover it, might be a good spot.

But diluted essential oils are different: “It’s important to note that all essential oils should be diluted in a carrier oil and never used directly on the skin or a chemical burn can occur,” says Murray. “That being said, apply a small amount of diluted essential oil to your forearm or the crease of your elbow.”

Murray does note that an at-home test could have varied results. An example includes what state your skin is in at the time. If someone has sensitized skin from the weather or overexfoliating, she says, the product might sting or cause redness.

Redness or burns can be due to a compromised skin barrier rather than an allergic reaction. “It’s not that they can never use the product,” explains Murray. “Once the skin barrier is healed, using that product may be possible!”

Most common allergens: what’s on the list?

Skin care is a little trickier and less transparent than a patch test. If you’re wondering what might cause problems, Murray has a list.

“Nut oils, sulfur (which can be found in acne/calming products), compounds found within certain essential oils such as linalool, citral, and eugenol,” she says.

But allergens are also unique. If you’re curious, check out the FDA’s list of common allergens found in cosmetics. One of these ingredients could be the culprit of your woes.

Most cosmetics are formulated to be safe for the general population, when used as directed. But this doesn’t mean it’s risk-free. You might notice irritation, redness, or itchiness on the patch test area, but it’ll be much smaller than the reaction that could have happened if you’d slathered the goods on your face or scalp.

If you are worried about making your skin worse, especially if you have a pre-existing skin condition, or want to know what specific ingredients are causing a reaction, then going to a doctor’s office is a good choice.

While ingredient-specific tests may have higher risks, such as “worsening of contact dermatitis, post-inflammatory hyperpigmentation, and very rarely anaphylaxis,” according to Landriscina, it’s a good idea to make sure those reactions happen in a controlled environment where a professional can immediately help you.

However, there is also a financial risk with patch testing. While most insurance providers will cover some kind of patch testing, Landriscina says that “prices will vary greatly based on the type of test done and how many allergens are tested.”

Before you go in for patch testing, make sure you talk to your insurance provider to see what’s covered.

While immediately incorporating new products may feel like a shortcut to a glow-up, taking the time to add new products slowly and patch test each one will cut out detours like breakouts, allergic reactions, and the old “which product was it?!?”

Remember, the key to healthy and happy skin is patience.

Patience is also important when patch testing at a doctor’s office, because the process may not lead to an immediate revelation. “I’ve told many a patient that sometimes patch testing will leave you with more questions than answers,” says Landriscina.

“For instance, it may turn up no allergy at all — this helps to further narrow down the diagnosis. It can also turn up many allergies leaving one to wonder which of them is relevant. A good patch testing provider will help guide you through the results and provide you with information and strategies to avoid any associated allergens.”

So whether you’re at home or in a doctor’s office, keep it slow and steady. Your glowing, healthy skin is just around the corner.