A patch test is a diagnostic tool used by a dermatologist to identify the specific allergens or chemicals that triggers an allergic response leading to a form of dermatitis or eczema called allergic contact dermatitis. Patch tests are not the same as skin prick tests which are mainly used to diagnose hay fever. Skin prick tests are not useful in patients with rashes.
All patients that have eczema that is suspected to be of an allergic origin or eczema that cannot be controlled with usual medication or that is aggravated every time when a patient uses certain substances or is in a particular environment should be referred for patch testing.
The dermatologist will apply small amounts of common allergens or test substances in chambers that are applied in direct contact to the skin. This is usually on the upper back. They are fixed with a hypo-allergic plaster or tape. They are left in contact with your skin for 48 hrs.
The dermatologist will read the test in 48hrs and again in 72hrs, during this period the patient’s back is not supposed to come in contact with any water. At the 48hrs visit, the allergens will be removed from a patient’s back.
The allergens in the patch test series cover a lot of substances including metals, substances found in cosmetics and perfumes, substances in detergents and cleaning material, leather, plants, dyes, and certain types of foods amongst many. Sometimes patch tests may be personalized to a patient and the things a patient uses daily, be at home, or at work may be used to do the patch test.
The results of the patch test will help the patient to avoid the things they are allergic to. It also provides a list of other things that can cause a cross-reaction with the substances a patient is allergic to.