What Is Patch Testing? A Plain-English Guide
Patch testing is how dermatologists identify what your skin is allergic to. Here's what it involves, what it can find, and how to prepare.
What it is
Patch testing is the standard way to identify allergic contact dermatitis, the kind of skin allergy where you react to a specific substance you touch. Small amounts of common allergens are applied to your back under patches, and the skin is checked over several days.
How it works
- The patches go on (usually on the back) and stay on for around 48 hours.
- You return for readings, often at 48 hours and again a few days later, because reactions are delayed.
- A positive reaction shows up as redness or a small eczema patch at that allergen’s spot.
What it can find
Common culprits like fragrance, nickel, preservatives and lanolin. It identifies allergies, not irritation, so it is most useful when reactions keep recurring and you cannot pin down why.
How to prepare
Avoid heavy sun and steroid creams on the back beforehand, keep the area dry during testing, and bring a list of products that have caused problems.
Reading a label by eye, or using a free ingredient-checker, will tell you what is in a product. What it will not do is check it against the specific ingredients you react to.
To close that gap, a personal-list app like ClearaScan lets you save the ingredients you react to once and scan any product to flag only your triggers. It also keeps a Reaction Journal for flare-ups, a shared Care Circle so family or carers can scan for you, and a Trusted Products list for items you have cleared, and it is currently in early access. (Disclosure: our editor co-founded ClearaScan, and we are not paid to mention the others.)