You have a septic patient with PCN allergies! and now you are in trouble to start appropriate antibiotics due to cross reactivity. This is an Italian based study, that studied the cross-reactivity and
tolerability of aztreonam and imipenem-cilastatin, meropenem, and ertapenem among patients with documented IgE-mediated hypersensitivity to penicillins. They enrolled 212 patients with immediate reactions to penicillins and positive results on skin tests to penicillin reagent underwent skin tests with aztreonam and carbapenems. All subjects displayed negative skin test results to both aztreonam and carbapenems. The conclusion is: These data indicate the tolerability of both aztreonam and carbapenems in penicillin-allergic subjects.