Clinical data to support these findings include a study out of Italy which included patients with delayed onset reactions to penicillin.5
The researchers performed skin and patch testing on 214 patients and found that none had reactions to beta-lactams with dis-similar side chains to penicillin including cefuroxime, ceftriaxone, or aztreonam. However, 20% of penicillin allergic patients were also allergic to either cephalexin, cefaclor, or cefadroxil. These three cephalosporins all have similar side chains to either ampicillin or amoxicillin.
Additional research includes a “robust” study of 24 individuals who were indicated to have immediate allergic reactions to cephalosporins.6
The researchers performed skin tests of different cephalosporins on the individuals and found that out of the 24 individuals, a total of 9 had more than one cephalosporin allergy. Eight out of the 9 individuals all had cross-reactivity between cefotaxime, ceftriaxone, and/or cefuroxime. All of these antibiotics share a similar R1 chain (which amounts to a double-bond off a nitrogen, next to the beta-lactam base structure.)
In summation, the cross-reactivity between penicillins and cephalosporins with similar side chains appears to be about 20%. The cross-reactivity between similar cephasporin side chains appears to be about 40%.5,6
The recent data regarding carbapenem cross-reactivity appears to be a little more promising, a meta-analysis found that about 0.1% of individuals with a penicillin allergy will also react to a carbapenem.7,8
Of course, this data is not without limitations, such as the fact that only analyzed published literature. More definitive testing showed no patients with either immediate or delayed penicillin allergies reacted to skin or intravenous challenges of imipenem, meropenem, or ertapenem. This supports the idea that side-chains are the allergic determinants as carbapebems have dis-similar side chains to penicillin.
According to Dr. Jeffres, there is adequate data to show that skin, oral, and intravenous challenges show a lack of allergic reaction in patients with a penicillin allergy to antibiotics with dissimilar side chains (data on the penicillin to penicillin class cross-reactivity notwithstanding.)