to determine the cause of a contact dermatitis
(skin inflammation) or other sensitivity reaction
Since sensitivity reactions can be caused by one or more of a large number of substances to which your body may be allergic, avoidance may be impossible unless the cause is known. To find out the cause, the doctor will apply a small square of gauze to the skin, usually on the upper part of the back. The gauze is impregnated with a purified extract of the suspected substance or substances. If, after about forty-eight hours, there is an allergic reaction to the substance(s) under test, it can be assumed that the cause of sensitivity has been identified.