Chapter 18 - Patch testing and the evaluation of contact allergy

Abstract

Allergic contact dermatitis (ACD) represents approximately 20% of all contact dermatitis and is considered one of the most frequent occupational skin diseases. The pathophysiology of ACD is complex, involving T lymphocyte–mediated hypersensitivity, an inflammatory cytokine milieu, and possible natural killer cell responses. ACD should be suspected in patients with acute to chronic eczematous or noneczematous dermatitis with ill-defined borders or in a particular recognizable distribution, along with the absence of other systemic manifestations. The most common distributions are scattered or generalized, hands, and face. Clinical history gathering is paramount in the diagnosis and subsequent management of ACD, delving into the patient’s social history, work and home exposures, hobbies, and personal product use since it becomes significant in identifying real-life, relevant contact allergens. Thousands of contact sensitizers and allergens have been implicated in ACD, including metals, fragrances, preservatives, surfactants, rubber components, topical medications, and plastic resins. Site-specific considerations are crucial since certain contact allergens more commonly affect specific body regions. Patch testing (PT) is critical in identification of contact allergens implicated in ACD. Several types of core PT series are commercially available, including the North American Contact Dermatitis Group series, the thin-layer rapid-use epicutaneous test panel, and the Core Allergen Series of the American Contact Dermatitis Society. Other supplementary panels include the metal series and cosmetic-based panels. Patch test technique and patient compliance are critical to a reliable reading and interpretation. Some allergens exhibit positive reactions at 48hours while others can have delayed reactions and this must be considered in the total number of PT readings. Determining clinical relevance is a necessary and highly important step in the final interpretation of the PT results. The mainstay of treatment for ACD involves avoidance of relevant contact allergens. Patient and/or caretaker education and thoughtful counseling are of utmost importance for long-term successful outcomes.