What is patch testing?

Used to determine allergens for allergic contact dermatitis, series of allergens applied to the upper back, removed after 2 days, on day 4-5 patient returns for results, positive reactions show erythema, papules or vesicles. Identification of specific allergen helps find products free of these allergens, it is the gold std for dx of ACD, indicated for any pt w/acute/chronic pruritic/eczematous or lichenified dermatitis sus of secondary ACD. If dermatitis is extensive it should be controlled before PT, can't perform it on a lesion already found, stop immunosupressive tx, or lower dose, refrain from sun tanning 2-4 weeks prior to PT, not done for pregnant/breast feeding pt.