There are 3 types of test so I'll give a quick overview of what you should expect:
1. They may draw a vial of blood (usually from your arm) for one of several different allergy blood tests.
2. The most common test is the Skin Pr
!ck or Scratch test which they usually do on your back by scratching your skin with a sharp plastic applicator and placing a drop of a possible allergen on the skin. They usually do quite a few of these - as many as 24-60 depending on how many things they're testing you for - After 20 minutes they check to see if the skin develops a red, raised itchy area (called a wheal).
3. Depending on the results of the Scratch test they may also do Intradermal tests in which they use a syringe to inject a small amount of an allergen into the skin on your arm. They're more sensitive than scratch tests, but are usually only used for a small number of allergens (or if they suspect you're allergic to something that you don't react to on the Scratch test).
Either way, you'll end up with a bunch of round red dots on your back or arm, along with some pen marks that they use to keep track of what's where, and you may have some really itchy spots that you'll want to put benadryl on after the tests (if you have a reaction).