Ring-shaped positive allergic patch test reactions to allergens in liquid vehicles.

In recent years, we have observed several "ring-shaped" positive allergic patch test reactions to allergens dissolved in a liquid vehicle, a more intense response at the periphery of the site of application than in the central part. The occurrence of such reactions was evaluated for formaldehyde, Kathon CG, hydrocortisone and hexamidine diisethionate. Possible explanations for such reactions include pressure and/or a capillary effect.