PSHP 2026 Residency Conference

Purpose: Despite structural dissimilarity between penicillins and piperacillin/tazobactam (P/T), the incidence of cross-reactivity is unknown. This project evaluated the incidence of P/T reaction in patients with documented β-lactam reactions.


Methods: This was a retrospective single-center cohort analysis evaluating patients with reported β-lactam reactions who received P/T from July 2022 to June 2025. Patients were included if they received at least one dose of P/T during this time and had a previously reported β-lactam reaction. Patients were excluded if they were less than 18 years old, the P/T order was not administered, or there was insufficient quality of data. The primary outcome was the compared incidence of P/T reactions between patients with documented penicillin class reactions versus patients with reactions to all other β-lactams and β-lactamase inhibitors. Secondary outcomes were the incidence of P/T reactions in the following subgroups: subclass of previous β-lactam reaction, previous reaction type classification, P/T duration, and number of previously reported reactions. Results were analyzed using a Chi-squared analysis and descriptive statistics.


Results: Of 183 patients screened for analysis, 164 were included. Previously reported β-lactam reactions were classified as IgE-mediated (30%), non-IgE-mediated (18%), adverse reactions (23%), and unknown (29%). There were 21 (13%) patients with multiple β-lactam allergies. Potential reaction to P/T occurred in 0/87 (0%) patients with penicillin class reactions and 1/77 (1.3%) patients with non-penicillin β-lactam reactions (p=0.286). The patient with a documented reaction to P/T had a history of developing hives to cefuroxime. The median [IQR] length of treatment for patients who received multiple doses in an encounter was 2 [1-3.5] days, with the most common reason for discontinuation being targeted antimicrobial therapy (47%). 


Conclusion: In patients labeled with a β-lactam reaction, there was minimal incidence of P/T reactions, with 1/164 patients (0.6%) experiencing a documented reaction. These results support the hypothesis that cross-reactivity would be low based on structural dissimilarity, including penicillin class allergies. Further research is warranted to further elucidate the safety of administering P/T in this patient population.