Allergy Testing

Patch Testing

The Delayed Hypersensitivity Patch Test or “Patch Testing” is a type of allergy testing done by dermatologists and some allergists. Patch testing looks for skin reactions caused by chemicals that are in direct contact with the skin.

On average, women use 12 personal care products a day, exposing themselves to 168 chemical ingredients. Men use six, exposing themselves to 85 unique chemicals.

– Environmental Working Group

These chemicals can cause contact dermatitis.

Worse, many are absorbed through the skin into our bloodstream.

Soaps may contain fragrance, preservatives and even antibiotics.
A single scent may contain 50-300 distinct chemicals.
Sprays can cause an airborne contact dermatitis, affecting the eyelids in particular.

Examples of Allergic Contact Dermatitis

  • Eczema in the skin in contact with jewelry items, due to contact allergy to nickel
  • Reactions to fragrances in perfumes and household items
  • Eczema under adhesive plaster, due to contact allergy to rosin 
  • Swelling and blistering of face and neck in reaction to permanent hair dye, due to allergy to paraphenylenediamine
  • Hand dermatitis caused by rubber accelerator chemicals used in the manufacture of rubber gloves
  • Itchy red face due to contact with methylisothiazolinone, a preservative in wash-off hair products and baby wipes
  • Fingertip dermatitis due to acrylates used in hair extensions and nail cosmetics.
  • Reactions after dental implants containing acrylates
  • Localized blistering at the site of topical medications such as antibiotics
  • Swelling and blistering on exposed sites (eg face and hands) due to contact with plants such as poison ivy or, in New Zealand, the Japanese wax tree Toxicodendron succedaneum.

List obtained from Dermnet NZ, a very nice online resource.

What is a delayed type hypersensitivity reaction?

Delayed type hypersensitivity reaction (DHR) is a type of allergic reaction. The most common example of a DHR is to poison oak. It takes an initial exposure to “prime” the immune system against the allergen. Once your immune system has labeled the compound as a threat, your body will react to it after future exposures. It takes a few days after the exposure to mount the immune response, and when it does, you experience an itchy, weepy, red rash that can sometimes even blister. It takes at least a week to get better- assuming you are no longer in contact with the allergen.

There are many different appearances of contact allergy. It can range from just itching, to bright red or even dry, thick and scaly in cases of long-term exposure. Usually a product elimination is the first step to identifying an allergy, but patch testing can be utilized to determine the exact chemical that is causing the problem.

Types of Reactions


The most common appearance is an itchy, weepy, red plaque at the site of exposure.

Image from Dermatology Atlas


Hives aka urticaria are smooth, red welts that are very itch and come and go over the course of hours. There are allergens that can cause hives on the skin although it is a less common presentation of contact allergy.


Lichenoid contact dermatitis is another reaction pattern in the skin. It looks like a skin condition called lichen planus but is triggered by an allergen.

The Procedure


Allergens are prepared ahead of time by our staff. Sometimes up to 85 allergens may be used. You will have specific instructions about what medicines and topical creams you can and cannot use prior to your appointment.

Day 1: Application

The patches are applied to your back on Day 1, which is usually a Monday. Your back must be clean, dry and there must be enough spaces without rash to apply the patches. You are not able to wash your back or do activities that might compromise the placement of the patches.

Day 3: Removal

On Day 3, which is usually a Wednesday, the patches are removed. The first read is done by the dermatologist. Reactions on the first read may be “irritant” in nature, meaning it is no a true allergic reaction at this time. After patch removal, we still ask you to avoid washing your back so that our markings stay in tact.

Day 5: Second Read

On Day 5, which is usually a Friday, the dermatologist will do the second read. This is usually the final read, although in some cases a third read will be done on Day 7. Afterwards, you will receive a handout about any positive reactions and instructions about how to avoid the chemical in the future.


Patient Guide to Patch Testing – SmartPractice

Contact Dermatitis – information from Dermnet

Contact Dermatitis Institute – Allergen Database

American Contact Dermatitis Society – Allergen Information

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