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

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Patch testing helps identify which substances may be causing a delayed-type allergic reaction in a patient, and may identify allergens not identified by blood testing or skin prick testing.

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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.

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Application of the patch tests takes about half an hour.

Tiny quantities of 20 allergens in individual round aluminium chambers are applied to the upper back. They are kept in place with special hypoallergenic adhesive tape. The patches stay in place undisturbed for at least 48 hours. After 48 hours later, the patches are removed.