The risk of active sensitization to PPD

Para-phenylenediamine (PPD) and para-aminoazobenzene are strong sensitizers. By the patch test procedure, the patient may be sensitized to these agents. Combined testing of para-compounds may increase the risk of active sensitization. We studied the % of positive patch test reactions and their relevance. In order to assess the risk of active sensitization, we compared the % of relevant reactions of both early (2/3 days) and late (7 days) reactions. We also compared the percentage of positive patch test reactions to PPD and their relevance if simultaneously tested with para-aminoazobenzene. We studied the patch test reactions to PPD in the routine series in 2058 patients. In a group of 678 patients we tested PPD and para-aminoazobenzene simultaneously. 4.3% and 3.1% of the patients reacted to PPD, respectively, with and without simultaneous testing with para-aminoazobenzene. We estimated the reactions as relevant in 21.1% and 39.7%, respectively, with and without simultaneous testing with para-aminoazobenzene. We considered none of the late reactions as relevant. We found a high proportion of relevant patch test reactions to PPD, but sensitization to PPD by the patch test procedure is a risk. We state that routine series should not contain PPD. The high number of irrelevant late positive reactions strongly suggests active sensitization. Moreover, PPD is not a ubiquitous allergen and can be tested on a non-routine basis if industrial exposure to para-compounds is suspected or if a specific localization (e.g., head or feet) prompts the testing of PPD. Testing PPD combined with para-aminoazobenzene does lead to a slight increase in positive reactions to PPD (p<0.25) and to an increase in irrelevant reactions (p<0.10).