How to patch test a new skincare product, and what it cannot tell you

Two tests that share a name

"Patch test" is used for two different things. At home, it usually means trying a product on a small area before using it all over: this guide calls that a home use test. In a clinic, patch testing is the standard procedure for diagnosing contact allergy: allergens are applied to the skin under covered patches in standardised conditions, and the results are read up to seven days later.¹

The home use test, as dermatologists describe it

The American Academy of Dermatology suggests these steps before adding a product to your routine:²

• Apply the product to a quarter-sized test spot twice a day for seven to 10 days, somewhere it will not be rubbed or washed away, such as the underside of the arm or the bend of the elbow.
• Use the normal amount and thickness, and leave it on as long as you normally would. For a product you wash off, such as a cleanser, the academy suggests five minutes or as long as the instructions say.
• If there is no reaction, such as red, itchy or swollen skin, after seven to 10 days, the academy says to go ahead and use the product.
• If the skin reacts, wash the product off gently as soon as possible and do not use it again.²

The academy adds that some ingredients, such as retinol and glycolic acid, can irritate skin, particularly sensitive skin, and calls this normal and temporary.² Some products carry test instructions of their own: the FDA notes that makers of certain hair dyes tell users to test a small amount first, and advises following the maker's instructions.⁵

Dermatologists use a standardised version, the repeated open application test. The European Society of Contact Dermatitis describes applications twice a day for up to 10 days to two weeks, sometimes up to four weeks, on the inner forearm near the elbow crease, because some allergies show only once a substance has built up in the skin through repeated exposure.¹

What a home test cannot tell you

A clear result is not proof. The same guideline notes that a negative result after one to two weeks of repeated application does not rule out a relevant contact allergy, and that even a negative patch test with a whole product does not exclude allergy to one of its ingredients, for example because that ingredient is present at too low a concentration.¹

A reaction, on the other hand, does not say which ingredient caused it, or whether it was allergy or irritation. The guideline distinguishes allergic contact dermatitis, an immune reaction to a specific substance, from irritant contact dermatitis, a non-specific response.¹ When the cause is hard to pin down, the American Academy of Dermatology suggests seeing a board-certified dermatologist, who may recommend patch testing.²

What the ingredient list adds

Read the list before the test, not after. The FDA calls knowing what you are sensitive to, and reading the ingredient panel for it, the best way to prevent an allergic reaction to a cosmetic.⁵ The European Society of Contact Dermatitis guideline says counselling after a diagnosis should name the allergen, its synonyms and related substances, and show the patient how to read cosmetic labels; with a formaldehyde allergy, for instance, the ingredients that release formaldehyde need avoiding too.¹

Fragrance is where the list is least complete. A scent mixture appears as one word, Parfum or Fragrance⁶, and in the US its components need not be named.⁵ EU labels must also name certain fragrance allergens above set levels, such as Linalool, Limonene and Citrus Limon Peel Oil (Annex III, entries 84, 88 and 353).⁶ The academy also cautions that products labelled unscented, or made with plant ingredients, may still contain fragrance-related ingredients.² EU fragrance allergen labels and cosmetic labels and sensitive skin cover what to look for.

Keep the packaging. The British Association of Dermatologists asks patients going for patch testing to bring their own products together with the packaging that lists the ingredients.⁴

When to see a doctor

• A severe reaction that does not ease: the American Academy of Dermatology says you may need a dermatologist to help manage the symptoms.²
• Itching or rashes that continue: a dermatologist may recommend patch testing to find the cause, and more than one substance can be involved.³
• Any suspected allergy: the FDA advises consulting a healthcare professional, as sensitivity to allergens can become more severe over time.⁵

A diagnostic patch test usually means three appointments in a week: patches go on, are removed after 48 hours, and the skin is checked again a few days later. It finds contact allergies, not food allergies, and a negative result is useful too, because it makes contact allergy a less likely cause.⁴

What to look for on the label

• Names you already react to, by their label names and synonyms.
• Parfum, Aroma or Fragrance, any named fragrance allergens after it, and scented plant oils.
• The product's own instructions, including any instruction to test before use.
• Claims such as "dermatologically tested": they describe the maker's testing, not your skin. See what skincare label claims mean.

This page explains what tests involve; it is not medical advice. The skincare ingredient checker marks fragrance and named fragrance allergens in a list you paste, so you can find the names you already know affect you. It cannot tell you how your skin will respond.