Patch Testing 101: The Process of Determining Skin Allergy Causes 

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:

• First appointment:To a sterilized back, the nurse will apply small quantities of anywhere from 25 to 100+ chemicals in individual small plastic or aluminum chambers taped in place by paper hypoallergenic tape.  These patches stay in place for 48 hours and cannot be disturbed or washed.
• Second appointment: Occurs 48-hours from first procedure. Patches are removed and a preliminary read is done.
• Third appointment:Occurs at least 48-hours after the second appointment. Back must be kept dry and free from perspiration to generate a valid result.

Patch Testing – FAQ

Why are you testing my back when my rash is on my face or hands? Why does it take so long?

1. Patch testing is asking your skin’s immune system whether its allergic to a certain substance.  The skin’s immune system is connected but entirely different than the body’s internal immune system in that it must “see” the substance on the skin — a process which can take longer than 96-hours.  The substance must penetrate the skin and then the immune cells in the skin assess it. If the substance is recognized and seen as allergic, then an attack ensures which results in a skin rash called dermatitis.

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?

1. Patch testing does not test for food allergies. Prick testing (see above for the distinction)

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?

1. No sun exposure on the back for two weeks before patch testing and do not get your back wet during the testing period (i.e. bathing, exercise, swimming).