The outcome of delayed patch test readings on day 7 in Thai population: A prospective analytic study

Objectives: This study aims to evaluate the outcomes of delayed patch test readings on day 7 compared to the commonly used day 4 protocol and to identify patient characteristics and allergens associated with new positive reactions observed on day 7. Ideally, patch test readings should be performed on three occasions: at 48 hours (day 2), at 72–96 hours (day 3 or 4), and at day 7. This schedule maximizes the detection of both early and late reactions, ensuring the highest diagnostic accuracy. However, in real-life clinical practice, three readings are often impractical. Most centers conduct two readings at 48 and 96 hours, which capture the majority of contact allergies. Many studies suggest that an additional reading around day 7 may detect 7–30% more contact allergies. To date, no study has evaluated this extended schedule in Thai population. Material and Methods: A total of 239 participants with suspected allergic contact dermatitis who underwent patch tests between June 2023 and September 2024 were enrolled in this study. Patch testing was performed using the Institute of Dermatology’s 32-allergen baseline standard series, supplemented by additional series tailored to the patient’s history and rash characteristics. The tested allergens were occluded for 48 hours, and readings were conducted on day 2 (48 hours), day 4 (96 hours), and day 7 (168 hours) after the application. Results were interpreted following the International Contact Dermatitis Research Group criteria. Results: Of the 239 participants, 28 (11.72%) demonstrated new positive patch test reactions on day 7 reading. The allergens with the highest proportions of the new positive reactions on day 7, in order, were potassium dichromate (60%), cobalt (57.14%), and p-phenylenediamine (42.86%). All delayed reactions exhibited a reaction strength of 1+. No significant associations were found between age, sex, underlying diseases, occupation, rash characteristics, and the positive patch test results on day 7. Conclusion: Our study showed that 11.72% of participants developed new positive patch test results on day 7 reading, consistent with findings from the previous literature. Delayed patch test readings are valuable for identifying additional positive reactions to certain allergens, such as metals. Limitations of this study include its single-center design and relatively small sample size.