Which initial diagnostic study is most appropriate for a patient with symptoms of contact dermatitis?

The most appropriate initial diagnostic study for a patient with symptoms of contact dermatitis is patch testing. This method is specifically designed to identify allergens that may have caused the dermatitis by applying small amounts of specific allergens to the skin and observing for a reaction over several days.

Patch testing is effective in diagnosing allergic contact dermatitis because it allows for the identification of delayed hypersensitivity reactions, which are characteristic of this condition. Given that contact dermatitis is often triggered by exposure to allergens, establishing what substances the patient is reacting to is crucial for both diagnosis and future prevention of exposure.

Skin scraping and microscopy are typically used for conditions like scabies or fungal infections, where a cellular or microbial cause is suspected; however, these studies do not help diagnose allergic reactions. Similarly, RAST testing, which measures IgE antibodies to specific allergens, is more relevant for immediate hypersensitivity reactions such as those seen in allergic rhinitis or asthma, rather than contact dermatitis. Skin biopsy is generally reserved for cases where a definitive diagnosis is needed for other dermatological conditions, but is not a primary tool for evaluating contact dermatitis. Therefore, patch testing stands out as the most targeted and informative initial diagnostic approach for suspected contact dermatitis.