Quantity of Allergen Per Unit Area Is More Important Than Concentration for Effective Patch Testing

Background: Concentration of a material in a patch test vehicle such as petrolatum has generally been considered of prime importance in obtaining successful patch test results. With the advent of newer patch test methods that allow strict control of both concentration and total quantity of an allergen in a patch, it has become possible to study the importance of total allergen quantity presented to the skin. Objective: This study was undertaken to determine if different quantities of allergen presented on patches at the same concentration would produce different patch test results. Methods: Nine individuals known to be allergic to potassium dichromate were retested with two patches, both containing a concentration of 175 ppm of potassium dichromate. The standard patch was seven times thicker than the “thin” patch and contained .88 μg/cm2 versus .13 μg/cm2 of potassium dichromate. Results: Six of the 9 patients reacted to the standard patch, whereas none reacted to the thin patch. Conclusion: Total allergen quantity in a patch available to the skin surface is more important than concentration of allergen in the patch test vehicle.