Severe immediate hypersensitivity: Implications of cross-reactivity between penicillins and cephalosporins
Antibiotics that do not share the same or similar R1 side-chain to penicillins (eg cefazolin) are considered safe to administer to hospitalised patients with severe immediate hypersensitivity to penicillins.
For directed therapy in patients with severe immediate hypersensitivity, an antibiotic that does not share the same or similar R1 side-chain (eg cefazolin) should be used if a beta-lactam antibiotic is the preferred option.
For empirical therapy in patients with severe immediate hypersensitivity, an antibiotic that does not share the same or similar R1 side-chain (eg cefazolin) can be used if a beta-lactam antibiotic is the preferred option. Factors to consider when deciding whether to use a cephalosporin include:
- the certainty of diagnosis
- clinical presentation – changes to the patient’s clinical condition (eg hypotension, tachycardia, respiratory distress) may cause diagnostic uncertainty because it may be difficult to differentiate between allergy and clinical deterioration
- data supporting the use of a cephalosporin.
Amoxicillin and ampicillin (often referred to as aminopenicillins) are the only penicillins that share the same or similar R1 side-chain with cephalosporins1. Cefalexin or cefaclor (often referred to as aminocephalosporins) should not be administered to a patient with severe immediate hypersensitivity to amoxicillin or ampicillin because they have similar R1 side-chains. Most cross-reactivity reported in the literature is between these drugs; rates of cross-reactivity between drugs that do not share a similar R1 side chain are exceedingly low.