Comparison of patch test positivity after 24 and 48 hours of occlusion time in patients of allergic contact dermatitis: A prospective study.

Background: Patch test is the gold standard for diagnosing allergic contact dermatitis. Conventionally, the patches are applied for 48 h, which in tropical weather conditions causes excessive sweating, leading to irritation, and sometimes the patches come off, making the test inconclusive.
Objective: To compare the patch test positivity after 24 and 48 h of occlusion time in patients of allergic contact dermatitis, using standard allergen concentration.
Materials and Methods: Clinically suspected patients of allergic contact dermatitis were enrolled and patch tested using the Indian Standard Series, parthenium acetone extracts (1:50, 1:100 and 1:200 dilutions) and patient material. Patches were applied in duplicate on either side of the back, using a random number table. One set of patches was removed after 24-h of occlusion, while the other set after 48-h. Readings were performed at 48- and 96-h by two independent dermatologists, blinded to the duration of occlusion.
Results: The study had 97 adult patients (58 males and 39 females; mean age: 48.12 ± 13.07 years). A total of 133 and 142 positive reactions were observed after 48 h occlusion at 48 and 96 h reading, respectively. Of these 117 (87.9%) and 132 (92.9%) patches were positive and concordant and noted at 24 h occlusion time. The Cohen's kappa coefficient were 0.94 for 48 h and 0.97 for 96 h reading, hence showing an almost complete agreement (ⱪ > 0.81) between patches occluded for 24 and 48 h.
Conclusion: Though there is no significant difference in patch test positivity among ISS allergens after either occlusion time, 48 h occlusion performs significantly better compared with 24 h, when reactions of all allergens (ISS, patient material and parthenium acetone extract) are analysed together.
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