Background: Despite general recommendations, patch test readings are still sometimes terminated at day 2.
Objective: The consequences of failure to read patch tests after day 2 and the influence of exposure time to the patches is quantified.
Methods: Patch test data from 9,946 patients with at least one positive reaction at day 2 or later and readings at both days 2 and 3 (from 21,062 patients tested between January 1, 1990, and December 31, 1993) collected by the Information Network of Departments of Dermatology in Germany were analyzed.
Results: Comparing results at days 2 and 3, 34.5% of all positive reactions appeared at day 3 only, whereas 8.3% initially judged (weakly) positive were not considered allergic at day 3. Some standard allergens showed specific patterns. Additional positive reactions and an increasing percentage of diminishing reactions can be observed at day 4 (compared with day 3).
Conclusion: The importance of late reading(s) is emphasized by the proportion of potentially false judgments based on readings at day 2 only.