what is patch test

Patch testing is a skin test used to find the cause of a possible allergic reaction on the skin, also known as allergic contact dermatitis. It is a scientific method of investigation that involves applying diluted allergens to the skin, usually on the back for convenience, for 48 hours. The patch tests are then read at 96 hours as reactions usually take 48–96 hours to develop. Positive reactions produce a patch of dermatitis at the application site of the offending allergen, which will appear as a red and possibly raised, vesicular and even blistering area. Patch testing is different from skin prick tests, which are used to diagnose hay fever allergy. During patch testing, it is best to avoid potent topical steroids to the application site for three days before patch testing is commenced. The results from patch testing can help uncover what’s causing your skin to react, and once the cause (or causes) of your contact dermatitis is known, a plan can be created to help you avoid what’s causing your rash.