At any rate, this is a literature review that aims to update the classical teaching that cross-reactivity between cephalosporins and PCN is ~10%. They identified 406 articles on the topic and distilled it down to 27 respectable articles for inclusion in summary. They rate the quality of the articles, and, unfortunately, find only a few good or outstanding articles and a preponderance of adequate evidence.
But, essentially, what they find is the cross-reactivity boils down to the presence of a shared R1 side chain present on first-generation and some second-generation cephalosporins. Specific first-generation cephalosporins, such as cefadroxil (Duracef), were seen to have up to 28% cross-reactivity in some series, though the typical rate was lower, down to 0.11% with cefazolin (Ancef). The largest meta-analyses estimated the true cross-reactivity at ~1% rather than 10%, with most of these occurring with first-generation cephalosporins.
In summary - 3rd-generation and greater cephalosporins with disimilar R1 side chains can probably be used in appropriate clinical situations despite a PCN allergy without incidence of allergy greater than in those patients who do not have a documented PCN allergy.
"The use of cephalosporins in penicillin-allergic patients: A literature review."