PTBP‐FR and TRUE Test®

To the Editor, A recent article on p-tert-butylphenol-formaldehyde resin (PTBP-FR) in TRUE Test® suggests that this patch may be sensitizing patients on whom it is tested (1). As the manufacturer of this product, I would like to point out a few additional considerations. As part of a study on the association between nickel allergy and orthodontic appliances, Stenberg et al. noted that 0.5% of 1000 upper secondary pupils tested with TRUE Test® had late reactions that the authors interpreted as possible active sensitization to PTBP-FR (1). Of these 5 patients (all female), 4 were atopic, and 3 of these had a history of atopic eczema. Six months before testing, one of the latter subjects (subject 1) had purchased a watch strap to which she had developed an eczematous reaction. Although the authors stated in the brief case report that the strap was sent for chemical analysis, they did not identify the contents, and indicated that the results ‘possibly indicated active sensitization to the PTBP-FR’ (1). From the results of the gas chromatographic–mass spectrometric analysis, we later learned that PTBP-FR monomers were present in the glue of the watch strap, which the authors estimated contained 80 mg of PTBP-FR. Not until the Discussion did the authors note that this subject’s reaction could have been a normal late reaction. Curiously, however, they instead concluded that the history of reactions to the strap were in support of the patient being actively sensitized by testing. Upon retesting, this patient had a positive reaction (++) to PTBP-FR on D7, but she was not seen at 72 or 96 h, the standard for confirmation of active sensitization. Consequently, another normal late reaction or reactivation of an existing allergy, as suggested by her eczema being associated with the watch strap, cannot easily be ruled out. Eliminating this patient’s questionable data from the analysis reduces the frequency of active sensitization in this study to 0.4%. The results of 3 more subjects are also open to interpretation. Both subjects 2 and 4 reported a red square on their back 1 month after their initial testing. Both were retested and had positive (+) reactions to PTBP-FR on D3 and on D7. Interestingly, for subject 2 the authors concluded that the results indicated active sensitization, whereas for subject 4 they concluded that the result could indicate active sensitization but that a normal late reaction was equally possible. The clinical courses of these patients, as reported, appear to be almost identical; the inconsistency in interpretation was not addressed. Subject 3 had a doubtful reaction on D3 and a positive reaction (++) on D7. Again, the authors concluded that active sensitization occurred, although a normal delayed reaction is an equally valid interpretation for a doubtful reaction that later progresses. Removing these 3 patients from the analysis decreases the prevalence of active sensitization in the report to 0.1%. Interestingly, the rate of active sensitization to p-phenylenediamine 1% pet. was more than an order of magnitude higher (1.5%) when the German Contact Dermatitis Research Group (DKG) decided to remove this allergen from its baseline series (2). In the same study, the frequency of late reactions to epoxy was 0.3%, a level that was found to be acceptable. Consequently, not only is the rate of de novo sensitization from exposure to the PTBP-FR patch on TRUE Test® debatable, but so too is whether that low rate (0.5%), if accepted as valid, is objectionable. Because the dose per unit area when testing with TRUE Test® is 6–10 times lower than when testing with 1% PTBP-FR in pet., Stenberg et al. concluded that dose per unit area could not explain the suspected sensitization (1). They therefore compared the chemical analysis of the TRUE Test® PTBP-FR with previously analysed constituents, which served as reference materials. The concentrations of monomers, dimers and trimers tended to be higher in the former than in the latter. This variability between the two products raises an interesting related point, and underscores the known variability – as much as 10-fold – across the raw material used to make PTBP-FR and the resulting patch test allergen products that are available commercially (3). This lack of standardization is especially provocative given the decision by the DKG to remove PTBP-FR from its standard series as of 1 January 2014 because of its decreasing