The only way to definitively say you are allergic to something that touches your skin (i.e. skincare products, clothes, chemicals or jewelry) is with a procedure called patch testing.
Patch tests — not to be confused with skin prick tests that are administered by an allergist on the inner forearm and have limited to no value in determining the cause of skin rashes — are used to determine causative agents in hay fever, asthma or hives (aka: things you ingest or inhale). If you are allergic to something that touches you, the resulting reaction is called Allergic Contact Dermatitis, which consists of redness, itching, burning, scaling and occasionally blisters. The irritation also lasts for weeks, not just hours or days.
Patch testing involves a minimum of three appointments, typically over the course of a week and follows the below procedure:
Patch Testing – FAQ
Why are you testing my back when my rash is on my face or hands? Why does it take so long?
In short, if you are truly allergic it does not matter where the contact occurs. The back is used as it is a large surface area and a lot of individual chemicals can be applied and read easily.
Can you test me for a gluten or other food-related allergy?
Tests for food allergies while patch testing asks your skin’s immune system if you’re allergic to a topical substance.
The chemicals we put on your back are determined by your common skin exposures. For example, a hairdresser is patch tested to the chemicals in haircare including perms and dyes while a mechanic would be tested for a different set of substances.
For patients with routine rashes, testing is typically done with what is called a standardized set of allergens/chemicals. In my office and hospital clinic I use the North American Contact Group Standard Series which consists of over 70 chemicals that is updated yearly based on the changing exposures of the Personal Care and Industry.
What can I do or not do before and during testing?