Background: There is an ongoing discussion on whether routinely patch testing with p-phenylenediamine (PPD) 1.0% pet. is safe, owing to the risk of patch test sensitization. Late-appearing patch test reactions may reflect patch test sensitization, but may also be attributable to a low degree of pre-existing sensitization.
Objectives: To follow the positive patch test reactions to PPD and its salt PPD dihydrochloride (PPD-DHC) in order to characterize reaction patterns concerning time and dose in PPD-sensitized individuals.
Methods: Volunteers with previous reactions to PPD 1.0% were included and patch tested with PPD and PPD-DHC in equimolar dilution series. There were then seven follow-up visits over a period of 28 days.
Results: Twenty-six volunteers completed the study, of whom 23 of 26 (88%) reacted to PPD 1.0%, and 69% reacted to PPD 0.32%. Altogether, 42% and 27% reacted to the corresponding equimolar concentrations of PPD-DHC. After day 7, no new reactions were observed to any concentration tested, either of PPD or of PPD-DHC.
Conclusion: No late-appearing reactions to PPD or PPD-DHC were observed at any dose. There is a risk of missing contact allergy when the dose is decreased.
Keywords: PPD; active sensitization; allergic contact dermatitis; contact allergy; delayed; dilution series; false-negative reactions; hypersensitivity; late-appearing patch test reactions.
© 2019 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.