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Tell me about Allergies!

Patch Testing


Patch testing is a way of diagnosing whether there are substances in your environment that are absorbed across your skin to cause allergy symptoms ranging from a rash or hives to chest tightness or difficulty breathing.  Possible allergens include virtually anything you come into skin contact with- hair and skin care products, cleaning and laundry products, jewelry and clothing, glues, topical medications. 

If your history suggests possible contact sensitivity, you may be advised to have patch testing.

Patch testing is different from allergy skin testing for inhalants (pollens, dust, cat, etc) or foods.  With these, the test is placed, a reaction occurs or doesn't occur within 20 minutes, and the test is read and interpreted then and there.   The allergens in patch testing cause a reaction over a 12-24 hour period or more- so the test substance has to be in contact with the skin longer.  To make this practical, here's how it's done: 

  • Test panels of common allergens are attached to medical adhesive paper- with several in each group. 
  • The panels are placed on the back, between the shoulder blades. They are flat and comfortable to sleep on. 
  • There are left in place for 2-3 days, and then removed. 
  • The first reading is performed then. 
  • A second reading is performed another 2-4 days later. 

Why two readings?  

At the time of the first reading, you will have had adhesive paper on your skin for 48 hours or so.  This alone can irritate the skin, even if there were no allergens present.  Sometimes tests that look positive at the first reading calm down and disappear before the second reading.  If this occurs, the response was probably inflammation and not true allergy.

During the time the patch tests are on, it is important to avoid:

  1. water: You'll have to shower carefully to avoid getting water on your back, and put off swimming
  2. any activities that will make you very sweaty: Don't plan to have the patch tests on during the weekend when you're playing a big softball game or running a marathon!

As with other types of allergy testing, you must stop taking antihistamines 5 days before the tests are placed.  (see list of medications to avoid).  If you've had to use steroid cream in the area over the week preceding testing, it's best to postpone testing to avoid false negative results.