dermatologic disorders
Contact Dermatitis
Diagnosis of Contact Dermatitis
Clinical evaluation Sometimes patch testing Contact dermatitis can often be diagnosed by skin changes and exposure history. The patient’s occupation, hobbies, household duties, recent travel, clothing, topical medication use, and cosmetics must be considered. Patch testingis indicated when ACD is suspected and does not respond to treatment, suggesting that the trigger has not been identified. In patch testing, standard contact allergens are applied to the upper back using adhesive-mounted patches containing minute amounts of allergen or plastic chambers (Finn chambers®) containing allergens held in place with porous tape. A standard contact allergen series consists of those allergens shown to be the most common in a particular geographic location (see table). It can be expanded to include additional substances as indicated (eg, a metal worker series, hairdresser series). Thin-layer rapid-use epicutaneous patch testing (T.R.U.E. TEST®) is a simple, easy-to-use kit with 36 of the most common contact allergens that can be applied and interpreted by any health care practitioner. However, it detects only about 50% of clinically relevant contact allergens. If available, patch testing with more extensive test panels is therefore recommended. The standard procedure is that allergen patches are applied to the skin on the back, left for 48 hours, and then removed. Skin under the patches is evaluated at 48 hours and at 72 or 96 hours after application for degree of erythema, size of the reaction, swelling, and vesiculation/crusting. A crescendo reaction (increase in the reaction from the first to the second reading) is typical for a positive reaction. False-positive results occur when concentrations provoke an irritant rather than an allergic reaction, but these can often be identified because they typically cause a decrescendo reaction (decrease in the reaction from the first to the second reading). False-positive results can also occur when reaction to one antigen triggers a nonspecific reaction to others or with cross-reacting antigens. False-negative results occur when patch allergens do not include the offending antigen or when patients have recently undergone immunosuppressive treatment. Definitive diagnosis requires a positive test result and a history of dermatits in the area where the tested allergen came in contact with the skin. 