Background Patients who suffer from acute IgE‐mediated allergy to a cephalosporin antibiotic are frequently assumed to be at high risk of allergy to other cephalosporins and penicillins. Aim To define cross‐reactivity patterns in patients with confirmed allergy to a cephalosporin. Methods Subjects presenting with a history of immediate allergy to a cephalosporin‐family antibiotic between March 2009 and July 2017 were investigated with specific IgE testing to penicillin, amoxycillin and cefaclor, followed by skin prick testing, intradermal testing and drug provocation testing with a panel of penicillins and cephalosporins. Results Out of 564 subjects with a reported beta‐lactam allergy, 90 identified a cephalosporin as their index drug. Fifty‐five (61.1%) of the 90 subjects tested had a history consistent with an IgE‐mediated reaction, of whom 24 (43.6%) were proven to be allergic to their index cephalosporin. Twenty (83.3%) of the 24 were allergic only to their index cephalosporin. Of the four remaining subjects, two were co‐sensitised to another beta‐lactam with a similar side chain, while the other two had no specific cross‐reactivity pattern. Major and minor penicillin determinants were negative for all cephalosporin‐allergic individuals. Conclusion In our cohort, cephalosporin allergy does not appear to be a class effect, with most cases found allergic only to their index cephalosporin. Co‐sensitisation to other cephalosporins or penicillins was uncommon, and when it occurred, was usually consistent with side chain cross‐reactivity.

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