Allergic contact dermatitis (ACD) describes a delayed type of hypersensitivity reaction that occurs when the skin comes into contact with a chemical to which it has previously been sensitized. A major development in ACD diagnosis was the creation of the patch test, which is credited to Josef Jadassohn and was first described as a method to understand mercury-induced eczematous dermatitis in a syphilis patient. Patch testing involves applying patches containing various allergens on the upper back of an individual, removing the patches, then interpreting the skin’s reaction to the allergens. While it is a brilliant technique, interpretation of the test results is yet to be perfected. Following are some aspects of this technique that require consideration, including the ingredients used, related legislative measures and testing limitations.
Clinical Relevance of Positive Patch Testing
Patch tests may signify an immunologic response but this response may have little or no clinical significance. Currently, when a patch test is positive, the patient’s history is further examined to aid in distinguishing whether this response is allergic or irritant in nature. The researchers then must ascertain whether the appropriate irritant controls are available, whether the morphology is that of ACD, whether the patient was previously exposed to the chemical, and a clinical course of action for once the chemical is removed. Furthermore, if the patient is exposed again to the putative allergen without being patch-tested, such as via a provocative use test, it must be determined if the reaction will be reproduced.
Clinical relevance algorithms have been offered by Marrakchi, Lachapelle, Ale and Hostynek to determine if another exposure will reproduce the allergic/irritant response. Enhancing the quantitative understanding of clinical relevance would allow mathematical evaluations to determine the probability of a true clinical allergy given a positive patch test result. Probability mathematics would also add credibility to diagnoses. In addition, a uniform approach with the appropriate follow-up in a large series of patients could pave the way for probability theory and databases to determine the likelihood of patch test-related clinical relevance. As many chemicals in standard and special allergen series are found in cosmetics and personal care products, it is important to be certain that an individual is indeed allergic to a chemical before advising them to avoid products containing that chemical.