Burdette, age 13, of Hutchinson, Kansas, for her question:

WHY DOES THE DOCTOR GIVE A PATCH TEST?

A patch test is given so the doctor can determine whether patients are allergic to certain substances. They can be used to identify the substances responsible for systemic allergies. Patch tests assist in the diagnosis and treatment of allergic disorders.

In patch testing a small amount of one or more substances suspected of being a sensitizer is placed on a small area (patch) of skin, usual sites on the back or the upper arm. The patch test is usually left in place for 24 to 48 hours.

If a skin reaction (redness, itching, irritation, tiny blisters) develops at the site during this period the patch test is positive, meaning that the person is sensitive to the test substance. If the test is negative, the person is not (at that time) allergic to the substance.

Although patch tests are very helpful in tracking down causes of allergies, they aren't 100 percent accurate.

Sometimes a patch test is administered as a precaution to determine in advance whether a person is allergic to a product before using it.