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The frequency of cross-reactivity between cephalosporins and penicillin is reported to be about 10%, although many experts believe it is much lower. Patients with penicillin allergies might be less likely to have reactions to cefazolin in particular, because it does not have the specific molecular structure that some other cephalosporins share with penicillin. In this meta-analysis, researchers evaluated the frequency of dual allergies to cefazolin and penicillin in 77 studies (>6000 patients).
The frequency of cefazolin allergy in patients with self-reported penicillin allergy was 0.6%, and the frequency in patients with confirmed allergy (verified by skin, in vitro, or in vivo testing) was 3%. In studies of the converse relation — penicill…