Cross-reactivity between penicillins and cephalosporins is approximately which percentage?

Cross-reactivity between penicillins and cephalosporins is approximately which percentage?

• A. 0%
• B. 0.5-1%
• C. 1-2%
• D. 5-10%

Answer: (C)

Cross-reactivity depends mainly on how similar the side chains of penicillins and cephalosporins are, not just on sharing the beta-lactam ring. Because modern cephalosporins often have different side chains than penicillins, the chance that a penicillin allergy will cause a reaction to a cephalosporin is quite small—about 1–2%. If the cephalosporin has a side chain very similar to the offending penicillin, the risk can rise a bit, but it remains low. This is why the best general estimate is in the 1–2% range; older, higher estimates (like 5–10%) overstate the risk.

Cross-reactivity depends mainly on how similar the side chains of penicillins and cephalosporins are, not just on sharing the beta-lactam ring. Because modern cephalosporins often have different side chains than penicillins, the chance that a penicillin allergy will cause a reaction to a cephalosporin is quite small—about 1–2%. If the cephalosporin has a side chain very similar to the offending penicillin, the risk can rise a bit, but it remains low. This is why the best general estimate is in the 1–2% range; older, higher estimates (like 5–10%) overstate the risk.