Patch testing in non‐immediate cutaneous adverse drug reactions: value of extemporaneous patch tests

BACKGROUND: Patch testing following a standardized protocol is reliable for identifying the culprit drug in cutaneous adverse drug reactions (CADRs). However, these patch tests (PTs) require pharmaceutical material and staff, which are not always easily available. OBJECTIVES: To evaluate an extemporaneous PT method in CADRs. METHODS: We retrospectively analysed data for all patients referred to our department between March 2009 and June 2013 for patch testing after a non-immediate CADR. The patients who supplied their own suspected drugs were tested both with extemporaneous PTs and with conventional PTs. Extemporaneous PTs involved a nurse crushing and diluting the drug in pet. in a ratio of approximately one-third to two-thirds. Standardized PTs were performed according to guidelines, with commercial drugs diluted to 30% or with active ingredients diluted to 10%. We analysed the data for the two PT methods in terms of the number of positive test reactions, drugs tested, and type of CADR for patients in whom the two PT methods were used. RESULTS: In total, 75 of 156 patients underwent the two PT procedures, including 91 double tests. Overall, 21 tests gave positive reactions with the two methods, and 69 other tests gave negative results with the two methods. CONCLUSION: Our series yielded results similar to those of published series concerning the types of CADR and the drugs responsible. Our results suggest that, for CADRs, if a patient supplies a suspected drug but if the pharmaceutical material and staff are not available for conventional PTs, extemporaneous PTs performed by the nurse with the commercial drug used by the patient can be useful and reliable.