Delayed positive drug provocation tests to betalactams with flare-up reactions of skin tests sites.

Non-immediate allergic drug hypersensitivity reactions (DHR) to betalactams are attributed to T cell-dependent mechanisms, varying from mild/moderate reactions, such as maculopapular exanthema (MPE) or delayed urticaria, to more severe cutaneous adverse reactions (SCAR)[1] . Patch tests (PT) and/or delayed-reading intradermal tests (IDT) have been proposed for in vivo diagnosis of this allergic phenotype, with the latter being generally reported as more sensitive. When skin tests are negative and there is no contraindication, a drug provocation test (DPT) may be performed, to confirm or rule out the DHR. However, a positive DPT alone does not allow any insight into the mechanism of the confirmed DHR. In a previous analysis of DPT to BL, we have shown that in most cases, the reaction is elicited within 48 hours after the drug administration.