Allergic or Not: Final Interpretation of Doubtful Patch Test Reactions From the North American Contact Dermatitis Group, 2019-2020.

Background: Doubtful patch test reactions generally do not meet criterion for positivity in patch testing. However, the North American Contact Dermatitis Group (NACDG) allows for doubtful reactions to be coded with a final determination of "allergic/positive" based on the temporal pattern, appearance, known characteristics of the allergen, and/or other supportive patch test reactions. Objectives: To analyze NACDG data from the 2019-2020 patch test cycle to identify patterns in the interpretation and relevance of doubtful reactions. Methods: The frequency and proportions of doubtful reactions were tabulated and analyzed for patterns. Statistical analyses were limited to allergens with ≥30 doubtful reactions to ensure adequate sample size. Results: Of patch-tested patients, 31.9% (1315/4121) had ≥1 doubtful reaction. Of 2538 total doubtful reactions, 46% (n = 1167) had a final interpretation of "allergic/positive." The allergens with the highest proportion of doubtful reactions at the final visit were hydroperoxides of linalool 1% (4.5%), fragrance mix I 8.0% (3.9%), and cetrimonium chloride 0.5% (3.4%). Methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) 0.02% ( P  < 0.001), MI 0.2% ( P  < 0.001), nickel sulfate hexahydrate 2.5% ( P  = 0.001), and neomycin sulfate 20.0% ( P  = 0.003) doubtful reactions were more likely to be interpreted as allergic than nonallergic. Methyldibromoglutaronitrile/phenoxyethanol 0.2% ( P  < 0.001), oleamidopropyl dimethylamine 0.1% ( P  < 0.001), formaldehyde 2.0% ( P  < 0.001), cetrimonium chloride 0.5% ( P  < 0.001), benzophenone-4 (sulisobenzone) 10% ( P  < 0.001), iodopropynyl butylcarbamate 0.5% ( P  < 0.001), cocamidopropyl betaine 1.0% ( P  = 0.002), and benzisothiazolinone 0.1% ( P  = 0.012) doubtful reactions were less likely to be interpreted as allergic. Of the 1167 doubtful reactions interpreted as allergic, 84.9% had current relevance. Conclusions: Doubtful reactions were common and approximately one half were coded with a final interpretation of "allergic/positive." Of those, most were clinically relevant. MCI/MI, MI, nickel, and neomycin were more likely to be interpreted as allergic.