What is Patch Testing?
Patch testing is useful for the evaluation of allergic contact dermatitis. This test is used to identify whether a substance that comes in contact with the skin is causing inflammation of the skin (contact dermatitis.) There are essentially two types of contact dermatitis: irritant contact dermatitis and allergic contact dermatitis.
Irritant Contact Dermatitis
An irritant substance is one that would cause inflammation in almost every individual if it was applied in sufficiently high concentration for long enough. An irritant reaction is caused by the direct contact of an irritant substance with the skin.
Allergic Contact Dermatitis
An allergic reaction is specific to the individual and to a substance (or a group of related substances) called an allergen. Allergy is a hypersensitivity (oversensitivity) to a particular substance, and always involves the immune system. All areas of skin that are in contact with the allergen may develop the rash. The rash will disappear if you avoid contact with the substance.
Why is Patch Testing Done?
If you have a dermatitis that started recently or if you have a persistent or unusual eczema, your allergist may suspect you have an allergic contact dermatitis. Patch testing is commonly advised for people who handle chemicals regularly, or on people who have seasonal or patterns of rashes.
How is Patch Testing Done?
Your Allergist Will Ask You a Series of Questions
First your doctor will discuss your skin problem with you. Subjects include:
- The site where your rash began and how it developed
- The treatments you have tried
- Previous skin disease
- The general health of you and your family, especially any tendency to get one or a combination of asthma, hay fever, or eczema
- Cosmetics and toiletries used
- Your occupation – this will focus on materials used at work and the effect of weekends and holidays on your dermatitis (if it settles during these times, it is likely that you are in contact with an allergen at work.) If other workers are affected with a similar rash then tell your doctor.
- Your hobbies or seasonality of the rash
- Foods you feel may be aggravating your dermatitis
If you can think of anything that you were in contact with around the time the rash first appeared, tell your doctor. Do not assume that just because you have been using something previously without a problem, it will not be the cause. Sometimes a cosmetic that you have been using for some time can become the cause of a rash. Your doctor will then examine your skin. The dermatitis is usually most severe at the site of exposure, but can be widespread (for example, if a patient with an allergy to a substance in nail polish touches her face, the dermatitis may spread.)
Your Allergist Recommends Allergens to be Tested
Your allergist will suggest for which allergens you should be tested. The standard selection of allergens used consists of the majority of allergens that are implicated in contact dermatitis. Together, the allergens in this panel cause over 85 percent of all allergic skin reactions. In addition, the allergist may suggest additional patch tests using other allergens specific to your occupation or site of the rash as well as your own cosmetics. Foods and airborne allergens can also cause contact reactions and are used in certain situations to evaluate specific rashes.
Conducting the Patch Test
Patch testing should be done on a skin site where the dermatitis is not apparent. The allergens are mixed with a non-allergenic material (base) to a suitable concentration. They are then placed in direct contact with the skin, usually on the upper back, within small aluminum discs. Adhesive tape is used to fix them in place, and the test sites are marked. The patches are left in place for 72 hours.
Preparing for Patch Testing
It is important that you are not taking any steroid medications by mouth (such as prednisone) within the month of your patch test. You should not receive a cortisone injection within one month of your test as well. Do not apply lotions or powders prior to the application of your patch tests. In addition, it is very important you are not applying any cortisone-containing creams to your back within two weeks preceding your patch test. Antihistamines may be taken if needed, or unless otherwise instructed. Avoid sun exposure to your back for two weeks preceding your patch test. The test may possibly stain clothing. Choose your clothing carefully with this in mind. You must not be pregnant or nursing.
What to Expect at the First Visit:
- Application of the patch tests (approximately 40-80 minutes)
- This is when the application of the patches is done.
What to Expect at the Second Visit:
- First reading after 48 hours (approximately 90 minutes)
- This first reading will allow us to assess the degree of the reactions and remove any significantly positive patches.
What to Expect at the Third visit:
- Delayed reading (approximately 20-40 minutes)
- Oftentimes a delayed reaction occurs. This means that during your third visit, a substance that was originally negative has since produced a response on your skin. At this visit you will be informed of the compounds that showed reactions. This will be reviewed with you, and a list of safe products or products that these compounds are in so you can avoid them will be emailed to you.
What Should I Do if A Patch Test Reaction is Positive?
- You should be given detailed information about sources of the allergen
- Carefully avoid any further contact with the allergen
- Carefully read ingredients of new products, especially cosmetics
- Use barrier creams and protective clothing to avoid the allergen
- If the allergen is at work, then discuss the options with your employer. They should provide materials to protect you from the allergen, or, if this is not possible, consider how to change your work.