A patch test is a diagnostic method used to determine which specific substances cause allergic inflammation of a patient's skin.

It is intended to produce a local allergic reaction on a small area of the patient's back, where the diluted chemicals were planted.

The chemicals included in the patch test kit are the offenders in approximately 85–90 percent of contact allergic eczema and include chemicals present in metals (e.g., nickel), rubber, leather, formaldehyde, lanolin, fragrance, toiletries, hair dyes, medicine, pharmaceutical items, food, drink, preservative, and other additives.

The APC then travels to a lymph node, where it presents the displayed allergen to a CD4+ T-cell, or T-helper cell.

This starts a complex immune cascade leading to skin inflammation, itching, and the typical rash of contact dermatitis.

Tiny quantities of 25 to ~150 materials (allergens) in individual square plastic or round aluminium chambers are applied to the upper back.

The back is marked with an indelible black felt tip pen or another suitable marker to identify the test sites, and a preliminary reading is done.

[citation needed] Patch Testing for cosmetic and skincare products can be broken down into a variety of different categories, including the following:[4]

Nickel allergy is more prevalent in young women and is especially associated with ear piercing or any nickel-containing watch, belt, zipper, or jewelry.

Allergens such as nickel, balsam of Peru, parabens, sodium benzoate, or cinnamaldehyde may worsen or cause skin rashes.