Patch Testing for Allergic Contact Dermatitis
Your physician thinks that your eruption may be caused or worsened by substances that come in contact with your skin. Water, solvents, soaps, cleaning products, acids and alkalis will damage anybody’s skin, causing irritant contact dermatitis, if contact is repeated often enough. Allergic contact dermatitis is rarer and is triggered by an abnormal reaction of the immune system: it may be caused by usually “inert” substances such as cosmetics, metals, rubber, external medicaments, etc. Patch testing is a time-honored method of discovering the cause of allergic contact dermatitis.
How it is done:
We test to the 65 most common Skin Allergens. The technique is different from conventional allergy tests used for hay fever, sinusitis, and asthma or food allergies: there are no needles, pricks or scratches. The strips are applied on the upper back. Their edges are inked with a hypoallergenic skin marker. They are left in place for 2 days and then removed. Reading of the tests is done on the 5th day.
What you have to do:
- Your eruption should be gone or under good control when patch testing is done. Your upper back should be completely free of dermatitis.
- You should not have received cortisone shots within 2 weeks of testing. If you are on oral cortisone, the dose should be less than 20 mg of prednisone per day. You can continue cortisone creams as long as it is not applied to the upper back. You can also continue oral antihistamines.
- You have to keep your back dry for the next 4 days. Showers are not recommended, but you can take a bath if you are careful not to get your back wet. Avoid sweating and exercising.
- If the test strips become loose, re-attach them with surgical tape.
- Two days after the tests, on Wednesday, we will remove the test strips.
- If some spot becomes extremely itchy before Wednesday morning, localize it as precisely as possible, and ask someone to cut out the test that overlies it, leaving the rest of the strip in place.
- Come back for your return appointment on Friday AM. Thereafter, you can shower. The adhesive and the ink marks will slowly disappear with time. Do not use solvents to clean your back.
- Rarely, reactions occur later than on the 5th day: if this happens, please call us back.
What to expect:
For the first day, the tape will feel tight. After the strips are removed your back will feel sticky. One or more spots may become itchy; usually 24-48 hours after the patches have been applied. These areas may be red, swollen, or sometimes oozy when the tests are removed, and this reaction should increase over the next few days. Reactions that decrease or disappear within 2-3 days after the patches are removed represent irritation rather than allergy and can be disregarded.
- Your dermatitis may “wake up” if you develop a positive reaction to the substance that caused it.
- Uncommonly, reactions may persist a few weeks. They can be treated with cortisone creams.
- As they heal, positive reactions may sometimes leave brown or white spots on the skin. Most of these marks will fade slowly over time, but some may be permanent. True scarring is rare.
- Very rarely, people may develop an allergy to a substance used in the tests. This is usually manifested by the appearance of a positive reaction 2-3 weeks after the tests.
- Extremely rarely, an immediate, systemic reaction may appear to a chemical applied on the skin.
Remember to bring all of your cosmetic/topical products in the 3rd visit.