Our apologies, dear reader. It seems we have gotten ahead of ourselves. While we were writing about the Allergen of the Year, systemic dermatitis, and patch testing in children, we forgot to start with the basics. Let us remedy that. This is the first of a 2-part series addressing the basics of patch testing. In this article, we examine patch test systems, allergens, patch test readings, testing while on medications, and patch testing pearls and pitfalls. Let us begin!
Patch Test Systems
There are 2 patch test systems in North America: the Thin-layer Rapid Use Epicutaneous (T.R.U.E.) test (SmartPractice), which is approved by the US Food and Drug Administration for those 6 years and older, and the chamber method.
The T.R.U.E. test consists of 3 panels with 35 allergens and 1 negative control. The T.R.U.E. package insert1 describes surgical tape with individual polyester patches, each coated with an allergen film. Benefits of T.R.U.E. include ease of use (ie, easy storage and preparation, quick and straightforward application) and a readily identifiable set of allergens. The main drawback of T.R.U.E. is that only a limited number of allergens are tested, and as a result, it may miss the identification of some contact allergies. In an analysis of the 2015-2016 North American Contact Dermatitis Group (NACDG) patch test screening series, 25% to 40% of positive patch tests would have been missed if patch testing was performed with T.R.U.E. alone.2
Chamber method patch testing describes the process by which allergens are loaded into either metal or plastic chambers and then applied to the patient’s skin. The major benefit of the chamber method is that patches may be truly customized for the patient. The chamber method is time and labor intensive for patch preparation and application. Most comprehensive patch test clinics in North America use the chamber method, including the NACDG.
Patch test chambers largely can be divided into 2 categories: metal (aluminum) or plastic. Aluminum chambers, also known as Finn chambers, traditionally are used in patch testing. There are rare reports of hypersensitivity to aluminum chambers with associated diffuse positive patch test reactions,3,4 which may be more common in the pediatric population and likely is due to the fact that aluminum is present as an adjuvant in many childhood vaccines. As a precaution, some patch text experts recommend using plastic chambers in children younger than 16 to 18 years (M.R. and A.R.A., personal communication). Metal chambers require the additional application of diffusion discs for liquid allergens, and plastic chambers typically already contain the necessary diffusion discs. Finn chambers traditionally are applied with hypoallergenic porous surgical tape, but a waterproof tape also is available. To keep the chambers in place for the necessary 48 hours, additional tape may be applied over the patches.
In patch test clinics, many dermatologists use a standard or screening allergen series. An appropriate standard series encompasses allergens that are most likely to be positive and relevant in the tested population. Some patch test experts recommend that allergens with a positive patch test frequency of greater than 0.5% to 1% should be included in a standard series.5 However, geographic differences in positive reactions can influence which allergens are appropriate to include. As a result, there is no universal standard series. Examples of standard or screening series include the American Contact Dermatitis Society (ACDS) allergen series,6 North American Baseline Series or North American 80 Comprehensive Series, European Baseline Series, NACDG series,2 and the Pediatric Baseline Series,7 as well as many other country- or region-specific series. There currently are 2 major commercial allergen distribution companies—Chemotechnique Diagnostics/Dormer Laboratories (series, individual allergens) and SmartPractice/allerGEAZE (series, individual allergens, T.R.U.E.).