If you have a recurring rash or breakout of eczema in certain areas and your doctor suspects that it is being caused by a specific allergen, your doctor may recommend an allergy patch test. Not to be confused with the skin prick test, which is usually performed by an Allergist, the skin patch test is used only to diagnose a contact allergy, which is an inflammation of the skin cause by direct contact with an irritating substance. The allergic reaction can often be caused by chemicals found in cosmetics or nail polish, or by a natural substance such as nickel, commonly found in jewelry and belt buckles. If you are meeting with your physician to discuss your allergies for the first time, sometimes it is helpful to bring any products that are known to provoke a reaction.
At Rao Dermatology, we test for 64 individual allergens. An absorbent pad is soaked in each allergen and taped onto the skin for 24 to 72 hours. The interpretation of the results requires considerable experience and training. By the site of the application, the type of reaction, and the specific allergen, the physician will determine which allergen you are reacting to. We can then help you learn how to avoid your allergens and educate you on what to do when avoidance is not possible.
About the allergy patch test
Before your patch test, do not expose your back to the sun for four weeks. The entire patch test will require three visits with your dermatologist. The first visit, during which the test will be applied to the skin, will take approximately half an hour. The skin around the patch test will be marked with a felt-tip pen. The marks from the pen can sometimes stain clothing, so either wear old clothes or garments on which the ink will not show. The patches are usually left taped to the skin for 48 hours. During this time, you must keep your back dry to ensure that the patches remain adhered to the skin, so baths, showers, swimming, and unnecessary sweating must be avoided. If the patches fall off, the test will have to be repeated.
The second visit, which will be 48-72 hours after the patches have been applied, will involve removal of the patches and analysis of any reaction thus far. The pen markings may need to be refreshed. Although the patches will not be reapplied, you still must keep the test area dry. While some allergic reactions show very quickly, some can take several days to appear.
The third visit, which will be 24-48 hours after the second visit, will involve a final reading and analysis of the patch test. Your physician will determine which allergens have caused a reaction, and will often give you a list of things to avoid that contain that substance. Your doctor might also give you medication to use in the event that your allergens cannot be avoided and reaction occurs.