Background: Standardization of patch testing has been difficult to achieve.
Objectives: This study aimed to identify physical variations in patch test systems that could affect delivery of allergens.
Methods: We compared the volume, depth, and contact area of 21 patch test delivery systems. We also filled a variety of patch test systems with different volumes of liquid (ferrous chloride) and petrolatum (disperse blue) allergens to investigate coverage and extrusion.
Results: The depth of chambers varied from minimal to greater than 1 mm. Mean areas ranged from 50 to almost 350 mm2. In most chambers, even the largest volume of liquid (40 μL) seemed to be completely contained by each product's absorbent material. The amount of petrolatum required to provide 100% coverage ranged from 15 to 45 μL. The dose delivered, as defined by mg per cm2, varied more than 2-fold across the systems.
Conclusions: There are considerable differences across various patch tests. Different patch test systems likely do not deliver the same dose of allergen if the same volume of excipient is applied. Appreciating the differences between different patch test systems may help refine recommendations for the amount of allergens that should be applied to different patch test systems.