Contact Dermatitis

Introduction: Patch test haptens are expensive and their frequency of use varies.Occasionally they pass their expiry date, leaving a significant amount of unused chemical.This study aimed to assess out-of-date or expired hapten viability, by comparing their reactions to in-date haptens. Methods:The study was conducted in the patch testing clinics at the Skin Health Institute, Melbourne, Australia between 1 December 2021 to 31 December 2023.Out-of-date haptens were donated by four different dermatology clinics, some back to 2004.Patients were tested with in-date haptens as per the usual investigative process.If they reacted at 48 h and an out-of-date haptens was available for that chemical, the patient was consented and the out-of-date hapten applied.These tests were read at 48 h and then assessed at 96 h, either in-person or with photos.Results: 16 out-of-date haptens were tested on 5 occasions.The overall percentage of out-of-date haptens patch test positivity in comparison to the in-date standard allergen was 83.8%.A total of 9 out-of-date allergens reacted with 100% positivity compared to their matched in-date haptens; these included nickel, cobalt, colophonium, p-paraphenylene diamine, toluenediamine, methylisothiazolinone, fragrance mix 2, hydroxyethyl methacrylate and tetraethylthiuram disulphide.Reactions even occurred to 2004 haptens!47.5% (n = 38) of out-of-date haptens had the same strength as the in-date haptens and 8.8% (n = 7) had a greater strength of reaction as the in-date haptens.Conclusion: Many out-of-date haptens do remain viable and produce positive reactions on patch testing.These results warrant pursuing a larger study with more participants and an even larger range of haptens.Our findings will be important for healthcare systems with scant resources to know that their haptens are likely to remain viable for a period.It could also lead to more targeted donations of out-of-date haptens from wellresourced to less-resourced countries, facilitating patch testing.