Question: How does a skin allergy test work?

There are actually two types of skin allergy testing—one type is an immediate type because it is testing for TYPE I allergic reactions. The other type, which is much less well known to the general public is patch testing, which is a delayed type of allergy testing based on the fact that it is testing for TYPE IV allergic responses.

Many people are familiar immediate allergic responses, some have had it performed in the office of an Allergist/Immunologist. This tests for allergies to pollens, weeds, grasses, trees, animal dander and other similar types of reactions that will give an immediate response. It is also known as RAST testing and requires scratching the skin to give a serologic response. If a whelp develops, this means the patient has a positive response for that particular allergen. Patients who test positive on RAST testing can then go on to receive allergy shots on a routine basis in order to desensitize to their particular set of allergens over a period of time. Then the patient would be retested at certain intervals in order to determine whether allergenicity has been lost to those previously sensitized allergens, can discontinue the allergy shots going forward.

In Dermatology, we much more commonly see delayed TYPE IV reactions due to the nature of the skin. Something allergenic touches the skin, the receptors send a message to the via a series of messengers through the immune system and it takes hours to days for the message to return back to the skin saying “produce inflammation, irritation and itchiness” and eventual oozing or weeping of the skin which is pathognomonic or classically symptomatic for allergic contact dermatitis.

For this type of allergy, the skin testing best performed is patch testing. Patches are placed on the back and left in place for an extended period of time—typically 48 hours. The patient is instructed to avoid sweating and water on the patches so that they remain in place and can be read out at the time of removal in the dermatology office. A readout is performed in the dermatology office to determine how extensive or severe the allergy is to manufactured goods such as metals, rubber accelerants, formaldehyde releasers, preservatives found in common household goods and grooming products, and multiple other common items that North Americans come into contact with on a routine basis.

To try and determine the cause of an allergic contact dermatitis is nearly impossible for a couple of different reasons. Many times, the delay can be lengthy between the time of contact and the time of the onset of the rash and so the contacts between so many other items can make impossible to determine the offending agent. Additionally, patients can say “I have not changed anything” or “Nothing is new” and think this holds the answer to the problem, but I explain that the immune system was designed to be an constantly changing, and with those changes, errors can occur. In the case of allergic contact dermatitis, this can represent an error of the immune system which has now become sensitized to something which is not new. Lastly, manufacturers can change a single ingredient within a list of ingredients in order to save a single penny, and they may not overtly advertise that fact to the consumer. If only a few people react to that ingredient change, the masses will be none with wiser.

Thus, it is critical to patch test patients with allergic contact dermatitis just as it is critical to allow the mechanic to look under the hood of your automobile if there are problems with it. You cannot expect him to determine the problem without the proper testing. The dermatologist can be expected to teach avoidance of the allergen causing the allergic contact dermatitis without patch testing, but what a simple and straight-forward fix! And much faster than years of allergy shots!


For more information about skin allergy testing, you may call us at 214-373-7546.



Dermatologist, Ellen Turner MD.

Dr. Ellen Turner is a double board-certified dermatologist with practices in Dallas and Irving, Texas. She specializes in medical and cosmetic dermatology, as well as anti-aging and regenerative medicine.

To schedule an appointment, please call 214-373-7546.