Allergy Skin Tests Versus Patch Tests
Many people use the term allergy skin tests and patch tests interchangeably which is not accurate. They are two completely different tests that are used to test for different agents that cause different problems. Allergy skin tests are an important part of an allergy evaluation. They are used to identify specific allergens that trigger allergy symptoms such as Rhinits, conjunctivitis or allergic dermatitis. Often these symptoms lead to sinus complications and asthma if left untreated. After obtaining a history and performing a physical exam, patients will undergo allergy skin tests that include airborne allergens (such as pet dander, dust mites, mold spores and pollens) and food allergens. These tests are relatively painless and provide quick immediate answers. They are done using two methods.
The puncture tests or scratch tests are done using sharp plastic kits with drops of allergens that we touch the back area using them. After 15-20 minutes the tests are read measuring any raised areas to identify what the patient is allergic to. To complete the testing, any allergens that did not react on the back will be reapplied using the intradermal test method (which consists of injecting a tiny amount of the allergen under the skin of the upper arm and to wait 15-20 minutes for a flare and wheal type of reaction if the allergen is positive. For adults and older children intradermal testing is usually done to complete the allergy skin tests and combine all the results prior to discussing immunotherapy which is the gold standard for treating and curing allergies. For young children, the intradermal testing is deferred and at times screening laboratory tests are pursued to obtain accurate results regarding the child’s allergies, in particular for food allergies. Patients are to avoid taking any oral antihistamines such as Benadryl, Claritin, Zyrtec, Allegra, Xyzal and any antacids such as Zantac, Pepcid or Tagamet for five days prior to the scheduled allergy skin tests. These tests can be done the same day of the initial visit if the patient is not taking any of the above medications that will block the tests and render them invalid. Tests will take one hour to complete.
Patch tests are used to identify allergens that cause skin rashes, contact dermatitis, urticaria or eczema. They are used to identify triggers that cause delayed hypersensitivity reactions on the skin. Patch tests are scheduled to be done on Monday, Wednesday and Friday in that sequence. On Monday, the patch tests which consists of three panels (of substances such as nickel, paraben, rubber, other chemicals found in every day products) are taped to the back. Patients return after 48 hours for the first reading and the patches are removed. On Friday, patients return 96 hour later for the second reading. Any raised red areas are noted for positive reactions. Patients are instructed on proper avoidance of positive substances. Patients are expected to keep their back dry during the week of testing. They are to avoid oral corticosteroids and topical cortisone creams on their back. If needed for intense itching, oral antihistamines may be used.