Relevance of Doubtful (“Equivocal”) Late Patch-Test Readings

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Background:Doubtful late patch-test readings are often not considered clinically relevant. As a result, allergens with doubtful late readings may not be disclosed to the patient, and relevant allergens may not be avoided. We hypothesize that doubtful late reactions at a day 7 reading may be relevant.Objective:We compared the patients' assessments of relevance to the strength of reaction at the final reading 168 hours after patch test application to determine if doubtful late patch-test reactions are relevant.Methods:Four hundred seven subjects who underwent patch testing from January 2005 to February 2008 were mailed a survey asking if their dermatitis was more than 80% better than before patch testing and whether they needed to avoid each of their positive patch-test allergens to remain free of dermatitis. Clinical charts of respondents who indicated their dermatitis was more than 80% improved were reviewed for strength of patch-test response at day 7 for each positive patch-test result, which was then compared to the patients' assessments of relevance formulated by the investigators' interpretation of the survey.Results:On the basis of survey interpretation, 63% of negative (0) late readings, 79% of doubtful (?+) late readings, 85% of + late readings, 84% of ++ late readings, and 100% of +++ late readings were considered relevant.Conclusion:Study limitations included small sample size, lack of formal validation of questionnaire, and response bias. However, based on the patients' assessments of relevance, allergens with doubtful late reactions were considered relevant almost as frequently as were allergens with + late readings.

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