Patch tests in non‐immediate cutaneous adverse drug reactions: Late readings on Day 4 is more sensitive than on Day 3

Background Patch tests (PTs) are recommended to identify the culprit drug in non‐immediate cutaneous adverse drug reactions (NICADRs). We recently reported that, in patients with NICADRs, a unique reading of PTs at day (D)2 compared with an additional second late reading at D4 missed almost half (45.3%) of the positive PTs. Objectives To assess the change in sensitivity of the PT reading on D4 compared with the reading on D3. Methods We performed a retrospective (July 2020–June 2023) monocentric study of patients who had PTs with two readings for a NICADR. We compared reading on D3 and the second reading on D4 for the suspected drug (primary outcome) and for the related drugs tested simultaneously (secondary outcome). Results During the study period, 249 patients underwent patch testing with D3 and D4 readings. Regarding the primary outcome, the first reading at D3 was positive for 13.7% of patients, and the reading at D4 for 24.9% of patients (p