Value of photo assessment in late patch-test readings—A multicenter study from six European patch-test clinics

Background: It is recommended that patch-test readings include a day (D)7 reading. Substitution of the D7 reading with a photo may be a valid option. Objectives: To compare the sensitivity of digital photos at D7 to clinical readings, to assess the number of positive reactions appearing at D7 only (late reactions), and after D7 only (delayed reactions). Methods: Patients patch tested in six European clinics were instructed to forward photos of the patch test reactions to the respective clinics at D7 (before attending the clinic) and at D21. Only allergens in the baseline series or TRUE Test were included in the data analysis. Results: Two hundred ninety-three of 629 patients had a total of 599 positive reactions, with 6.3% occurring at D7 only. When substituting the D7 reading with a photo (90% submitted), 26.3% of late reactions were missed and nine false-positive reactions were found. Delayed reactions were detected in four patients at D21 (65.3% submitted). Conclusion: Our data show that if the D7 reading is not performed, 6.3% of positive reactions from the baseline series would be missed, and if substituting the D7 reading by digital photo, 26.3% late reactions would be missed. Delayed reactions seemed rare.