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

INTRODUCTION

Patch testing is the gold standard for diagnosing allergic contact dermatitis (ACD). According to the European Society of Contact Dermatitis guidelines, at least two patch test readings are recommended ‒ at 48 hours (day 2), 72– 96 hours (day 3 or 4), and around day 7 to maximize the detection of both early and late reactions, ensuring the highest diagnostic accuracy.^[1] However, in real-life clinical practice, performing three readings is often impractical. Most centers conduct two readings at 48 and 96 hours, which capture the majority of contact allergies.^[1,2] Nevertheless, multiple studies suggest that an additional reading around day 7 may detect 7–30% more contact allergies.^[1,2] A delayed reading on day 7 has been reported to identify late positive reactions, particularly to certain allergens such as metals (cobalt and chromate), topical antibiotics, and corticosteroids.^[3-6] Factors associated with delayed positive reactions include age, sex, duration of contact with the allergen, underlying diseases, occupation, and rash characteristics.^[4,6-8]

To date, no study has evaluated this extended reading schedule in the Thai population. This study aims to compare the results of delayed patch test readings on day 7 with the standard day 4 protocol and to identify patient characteristics associated with positive reactions on day 7. These findings are crucial for preventing false-negative results and improving diagnostic accuracy, which may influence patient treatment guidelines.

MATERIAL AND METHODS

RESULTS

A total of 239 participants (77.15% female, 22.59% male, mean age 43.76 ± 15.16 years) were sequentially patch tested with the Institute of Dermatology’s 32-allergen baseline standard series, supplemented with additional allergens as needed. The demographic data of the participants who had at least one positive reaction are shown in Table 1. Of the 239 participants, 140 (58.58%) showed at least one positive reaction on day 4 and/or day 7, while 28 (11.72%) demonstrated new positive patch test results on day 7 reading. Participants with new positive reactions on day 7 had a mean age of 43.71 ± 16.23 years. Most of them had no associated underlying disease, including atopy, and no occupation related conditions. The head and neck were the most affected areas of the rash. A list of positive patch test allergens with the prevalences of positive reactions on day 4 and/or day 7 is shown in Table 2. A total of 23 allergens were associated with positive patch test reactions in our cohort, with nickel being the most frequently positive allergen. The allergens with the highest proportions of new positive day 7 reactions relative to the total positive reactions were, in order of frequency, potassium dichromate (60%, 3/5 patients), cobalt chloride hexahydrate (57.14%, 4/7 patients), and 4-phenylenediamine (PPD) (42.86%, 3/7 patients). All delayed reactions showed a weak positive (+) result.

DISCUSSION

Previous studies indicated that 6.3–15% of reactions would be missed without delayed patch test readings, and some have shown that an additional reading around day 7 may detect 7–30% more contact allergies.^[1,2,4,7] Many experts suggest extending patch test readings to certain groups of allergens, including metals (e.g., nickel, cobalt, and chromate), topical antibiotics (e.g., neomycin and bacitracin), and corticosteroids.^[3-6] Neomycin sulfate has been the most frequently reported allergen associated with delayed positive reactions.^[4,6,8] Nevertheless, the results from our study show only one case of a new positive patch test on day 7 to neomycin sulfate, and we did not observe late positive reactions to corticosteroids. This may be due to the small sample size of our cohort, resulting from the limited study duration.

Our findings demonstrated that the delayed patch test readings were useful for detecting reactions to metals, aligning with previous literature. The highest proportions of new positive day 7 reactions were observed for potassium dichromate (60%), followed by cobalt chloride hexahydrate (57.14%). Both allergens were considered statistically significant in late positive reactions. Other metals associated with late positive reactions in our study were nickel and mercury. Macfarlane et al. noted a 7.2% incidence of newly positive reactions after day 4. The allergens that most commonly produced late positive reactions were neomycin (26%), potassium dichromate (18%), and cobalt (6%).^[8] A study by Higgins and Collins found that 26 out of 203 patients (12.8%) had new positive reactions on day 7. Mercury and cobalt were the metals involved in these reactions. One patient with a late positive reaction to mercury had a history of an oral lichenoid reaction, while two patients were referred from the oral medicine department for patch testing with the dental series.^[9] A study from Malmö, Sweden, supported the importance of late patch test readings, especially on day 7, in patients with oral lichenoid lesions to detect contact allergies to dental materials. In this study, mercury, nickel, gold, and cobalt were the allergens with the highest frequency of late positive reactions.^[10] Thomas et al. discovered that patch test readings on day 6 were particularly helpful in identifying new positive reactions to nickel, cobalt, chromium, and bone cement components in patients with a history of arthroplasty and suspected implant allergy.^[11]

PPD had the third-highest proportion and was significantly associated with late positive reactions in our study. It is a well-known strong sensitizer commonly found in permanent hair dyes and a contaminant in temporary henna tattoos.^[3,12,13] Our study found that 1.26% of individuals exhibited late positive reactions to PPD 1% in petrolatum, which is similar to the 1.5% reported by Hillen et al.^[13] However, we used a 48 hour occlusion time compared to the 24 hour occlusion time in the study by Manaskiatti W et al., which potentially enhanced penetration and allowed more time for delayed reactions to develop.^[14] Another study by Geier et al.^[15] using PPD at the same concentration and a 48 hour occlusion time, found late positive reactions on day 6 in 2% of participants, which was higher than in our study. Although PPD is a strong allergen, delayed weak reactions in patch testing can still be expected. These may be due to differences in individual immunological reactivity. Some individuals have a less vigorous T-cell-mediated response, which can manifest as a slower reaction.^[16] Moreover, age-related differences in immune function might also affect the timing of these reactions. In elderly individuals, decreased lymphocyte proliferation can slow the development of allergic contact reactions, leading to weaker responses.^[17,18] Other methodological factors – such as patch test concentration, duration of occlusion, skin barrier integrity, and timing of readings – can also influence the occurrence of late reactions and the intensity of responses.^[13] However, in our study, the patch test concentration, occlusion duration, and reading times were consistent across all participants. Therefore, late positive reactions to PPD may be attributed to individual reaction patterns and variations in skin barrier integrity.

MCI/MI was another allergen that showed a significant association with late positive reactions in our study. However, it is not a well-known sensitizer that produces late positive results. A study by Madsen and Andersen revealed that the most common preservative associated with delayed reactions was imidazolidinyl urea (28%), followed by thimerosal, diazolidinyl urea, and MCI/MI, with the frequency of delayed positive reactions ranging from 11% to 19%.^[6] Jonker and Bruynzeel reported an incidence of new positive reactions to MCI/MI on days 6/7 at 0.7%, with five reactions observed in 760 patients.^[19]

Contact allergies can be associated with various patient characteristics, such as age, sex, atopic dermatitis, and occupational dermatitis.^[4,6,7,8] Elderly people and women generally have a higher prevalence of ACD than younger people and men. This is likely because they are more frequently exposed to different kinds of allergens in the environment throughout their lives. Other factors, including occupations, social habits, and culture, can also elicit ACD.^[3,20,21] A recent study by van Amerongen et al. reported that new positive reactions were mainly seen in the 18–30-year age group but were not significantly associated with sex, atopic disease, and occupational dermatitis.^[4] Another study by Madsen and Andersen found more new positive reactions in women and patients aged over 40.^[6] Alterations in skin barrier function, whether genetic or acquired, such as in atopic dermatitis, can lead to increased exposure to chemicals and induce contact sensitization, especially to allergens contained in topical products.^[20,21] Nevertheless, a systematic review and meta-analysis concluded that no significant correlation was identified between atopic dermatitis and contact sensitization.^[22] In our study, no significant associations were found between age, sex, underlying diseases, occupation, rash characteristics, and positive patch test results on day 7.

CONCLUSION

Patch test readings at 48 and 96 h are sufficient to identify the majority of positive reactions. Our study showed that 11.72% of Thai participants developed new positive patch test reactions on day 7 reading, consistent with findings from previous literature in other countries. Delayed patch test readings are useful and valuable for identifying additional positive reactions to certain groups of allergens, particularly metals, and in specific conditions.