Cross Reactivity:

See the comprehensive B-lactam cross-reactivity chart for groups of ß-lactams that may cross-react.

Between penicillins:

• Cross reactivity between penicillins is related to the ß-lactam ring ± side chains.

 Between penicillin & cephalosporins:

• ~ 1% in patients who report a penicillin allergy

• 2.55% in those with a confirmed penicillin allergy

• Cross reactivity between penicillins and cephalosporins is related to the similarity of the side chain structure. 

• NB:  Cefazolin is NOT expected to cross-react with any penicillin or cephalosporin as it does not have a similar side chain as any other β-lactam.

Between penicillins and carbapenems:  

[Clin Infect Dis 2014;59:1113-22]

• 4.3% for any type of hypersensitivity reaction

• 2.4% for IgE-mediated reactions

Among cephalosporins, and between cephalosporins and carbapenems:

• The degree of cross reactivity is unknown but is thought to be very low.
  

Aztreonam:

(not commercially available in Canada)

• Minimal cross reactivity except with ceftazidime (common side chain).