Background: Patch test readings are usually performed on day 2 (48 hours) and day 4 (96 hours). However, reports in the literature identify delayed allergy to metals, corticosteroids, antibiotics, some preservatives, acrylic and methacrylic monomers and p-phenylenediamine.
Objectives: The aim of our study was to identify the benefit of performing a day 7 (168 hours) reading to identify relevant late positive reactions.
Patients/Methods/Materials: Two hundred three consecutive patients were patch tested to the British Society for Cutaneous Allergy standard series with additional test series selected according to clinical history and applied at the same time.
Results: Twenty-six patients (12.8%) had new positive reactions on day 7 (168 hours), with 28 relevant positive reactions to 21 allergens. These included mercury 0.5% (2/26); cobalt chloride 1% (2/26); colophony 20% (2/26); disperse blue mix 106/124 1% (2/26); preservatives (4/26) that included Methylchloroisothiazolinone/methylisothiazolinone, sodium metabisulfite, and diazolidinyl urea; fragrances (7/26); and gentamycin sulfate 20% (1/26).
Conclusions: These results confirm findings in the literature and support the argument for performing a day 7 reading (168 hours) to identify relevant late positive reactions.