Patch skin testing is generally done to see whether a particular substance is causing allergic skin irritation (allergic contact dermatitis). Patch tests can detect delayed allergic reactions, which can take several days to develop.
Patch tests don’t use needles. Instead, allergens are applied to patches, which are then placed on your skin and covered with tape. During a patch test, your skin may be exposed to 30 or more extracts of substances that can cause contact dermatitis. These can include latex, medications, fragrances, preservatives, hair dyes, metals, steroids, and resins.
After placement, you will wear the patches on your arm or back for 48-96 hours. During this time, you should avoid bathing and activities that cause heavy sweating. The patches are removed when you return to your doctor’s office. Red and irritated skin at the patch site may indicate a contact allergy.