Patch testing is used to diagnose allergic contact dermatitis.  This is a delayed form of hypersensitivity as apposed to typical allergies which are an immediate type of hypersensitivity. 
Poison Ivy and Nickel are common forms of contact dermatitis.  Other causes are industrial chemical, cosmetics, and preservatives often used in detergents or clothing. 

This test is done over 3 visits.  At the initial visit, patches containing potential allergens or irritants are placed on the skin – usually on the back.  The patient then returns to the clinic after 2 days at which time the patches are removed and a first reading is done.  At the third visit, the final reading is done.

While the patches are on – for the initial 2 days – the test area cannot get wet.  Showers, exercise or any activity that causes perspiration or shifting of the patches should be restricted.  During the test and for a few days after, there may be some itching or rash on the back.  This will resolve spontaneously and a topical steroid cream can accelerate the process.