How are patch tests applied?

The suspected allergen is usually placed in an appropriate vehicle at an appropriate concentration. The patch test allergens are usually purchased in prepared forms, but less common allergens can be prepared individually. The allergens are placed in special wells that are taped against the skin of the back for 48 hours and then removed. It is important to instruct patients to keep the testing area completely dry. This skin area is then examined 24 to 72 hours after the patch is removed for a reading. A strong positive reaction has erythema, infiltration, papules, and vesicles. A bullous reaction is extremely positive. Interpretation of patch tests and correlation with the clinical disease are complex and usually performed by dermatologists.