Dr. Rita Lloyd Emeriti and Dr. Margo Reeder at The Departmennt of Dermatology – University of Wisconsin, School of Medicine and Public Health
Allergic contact dermatitis is a bumpy, itchy skin rash that shows up after touching a specific substance.
Poison ivy is a very common cause of allergic contact dermatitis and can even cause blisters. Although reaction to poison ivy is common, there are many other chemicals in our daily lives that can cause an allergy for some people. These chemicals can be encountered at home, at work or both. Possible sources of exposure include personal care products, like shampoo, soaps, lotions, nail polish or sunscreens.
Other sources include jewelry, plants and industrial chemicals. The treatment for allergic contact dermatitis is avoiding the substance causing the allergic reaction.
It is often hard to know if you have become allergic to a chemical for several reasons:
- You can develop an allergy to a chemical that you have had contact with many times in the past and had no reaction.
- You don’t react at the time of exposure. It can happen hours to days later.
- The reaction lasts for about three weeks, so it might be something you contact infrequently.
What is patch testing?
Patch testing is the diagnostic test for allergic contact dermatitis. Patch testing determines allergies to chemical ingredients such as preservatives, fragrances, dyes and metals. Patch testing works by seeing if a small amount of the chemical causes a skin rash. These chemicals are placed in contact with the skin for 48 hours and then removed. A skin reading is done 5-7 days after the test is placed to determine if an allergy is present. During patch testing, we test for 75-150 chemicals at a time. The number and type of chemicals tested varies based on what you have been exposed to and the location of the rash.Patch testing does not detect allergies to pets, grasses, mold or food. These allergies do not usually cause skin rashes. Instead, these allergies can cause runny nose, sneezing, watery eyes and sometimes skin swelling. Allergies to pets, grasses, mold or food are diagnosed with skin prick testing by an allergist.
What to expect on your patch testing visits
Your provider will take a detailed history of your possible exposures. This might include exposures in both the home and work environment. Ingredients are selected for patch testing based on your exposures. Patch testing usually requires over appointments over a single week:
- First appointment (Monday): the nursing staff applies the patches to your back. If you have rash on your back, we might apply the patches to other sites, too.
- Second appointment (Wednesday): the patches are removed at 48 hours and a reading is performed.
- Third appointment (Friday): the final reading is performed. We will provide a list of your allergies and discuss how to avoid them. We will also provide a list of “safe” products to use on your skin.
It is very important for patients to keep the back dry during the week of patch testing. Moisture such as sweating or showering can cause the patches to loosen or wash off. The back has to remain dry until the final reading, even after the patches have been removed.
Patch testing may be less accurate if you are taking certain medications that suppress the immune system. These medications might include certain chemotherapy medications, prednisone or steroids, mycophenolate or methotrexate. It is important to discuss your medications with your doctor when scheduling the patch testing appointments.
Occupational Contact Dermatitis
When there is a concern that the allergy may be to a workplace exposure, we take an extensive history of what you might be exposed to at work and at home, and evaluate for both. To help us understand your workplace exposures, it is helpful if you can bring with you to your appointment the “Safety Data Sheets” for materials that have contact with your skin.
Sometimes we actually want very small quantities of chemicals or objects themselves, such as sawdust for a woodworker or coolant for a machinist. We will usually discuss this at your first appointment and ask you to gather items from the workplace, if necessary. If you come from a long distance, we try to do what we can to limit your driving and may try to arrange these items in advance.
We also often need to evaluate personal protective equipment like gloves. Bringing information and a sample of exactly what you wear is important. Uniforms and other protective wear may need evaluation as well.
If you have a “safety officer” who handles possible workplace health issues, it helps us a lot to have the name and contact information for that individual.
If you have filed for workman’s compensation, bring any forms with you that we need to fill out. We have permission to contact your employer with workplace rash issues if you have filed for workman’s compensation. You will need to sign for us to have this permission if you have not filed a claim with workman’s compensation.
For information about other skin conditions and finding a Doctor please visit UWHealth Dermatology.