Positive relationship - intensity of response to p-phenylenediamine on patch testing and cross-reactions with related allergens

Background. Hair dye exposure is the most common cause of sensitization to p-phenylenediamine (PPD). Cross-reactions with structurally related allergens occur. Objectives. It is suggested that a stronger patch test reaction (3+ rather than 1+) to PPD (usually tested as 1% petrolatum) is associated with an increased propensity for cross-reactions. In this article we will demonstrate this association. Methods. Of 230 patients with allergic reactions to PPD on patch testing identified during 2007-2012 from clinical records, notes for 221 were available for review. Data were collected regarding age, sex, and grade of reaction [International Contact Dermatitis Research Group (ICDRG) criteria] to PPD. Cross-reactions with the following allergens, found in our baseline series, were recorded: Disperse Yellow 3, N-isopropyl-N'-phenylp-phenylenediamine (IPPD), and caine mix. Having excluded 23 doubtful reactions, the reactions from 198 patients were further considered. Results. Of the patients, 75.3% (n=149) were female, and themean age was 48.6 years (12-82 years). Of the patients allergic to PPD, 16.6% (n=33) showed cross-reactions with one or more related allergens. Cross-reactions were seen in 16% with a grade of 1+, 14.5% with a grade of 2+, 28.6% with a grade of 3+ when PPD was tested 1% pet., and 50.0% when PPD was tested at 0.1-0.001%, arbitrarily considered to be 4+ (p=0.02; Cramer's V = 0.23). Conclusion. An increasing likelihood of reactions to Disperse Yellow 3, IPPD or caine mix was seen with increasing strength of patch test reaction to PPD. The clinical relevance of these cross-reactions is unclear.