Penicillin Allergy

• The majority (80-90%) of penicillin (PCN) allergy labels are inaccurate - this translates to 9 out of every 10 patients who self-report a PCN allergy are NOT TRULY ALLERGIC when assessed by PCN skin testing.
  • Why? Besides inaccurate documentation/misinterpretation, anti-PCN IgE antibodies do not persist over time with >80% reduction in 10 years
• The risk of anaphylactic reactions is extremely rare (i.e., 0.004% to 0.015%) in patients with a reported PCN allergy.
• The risk of cross-reactivity with other beta-lactams is based on similar side-chains (see cross-reactivity table below).
  • PCN cross-reactivity rate to cephalosporins is thought to be low (~1-10%) and is based on similar side chains (e.g., penicillin, ampicillin, amoxicillin, and cephalexin have similar side chains)
  • Cefazolin does not share any side chains with penicillin! 
  • PCN cross-reactivity rate to carbapenems is thought to be very low (<1%)
  • Cephalosporin cross-reactivity rate to other cephalosporins (~40%) is based on similar side chains (e.g., cefotaxime, ceftriaxone, cefuroxime, ceftazidime, and cefepime have similar side chains)
• Cross reactions to monobactams (aztreonam) do NOT appear to occur
  • Exception: Ceftazidime and aztreonam share a similar side chain, so a patient with an allergy to one may cross-react to the other agent

• Almost always occur within 1 hour of administration

• Almost always occur after 72 hours of administration and can occur days to weeks after administration
• Rashes sometimes go away despite continued treatment

Stevens-Johnson Syndrome - exfoliative dermatitis with mucous membrane involvement

• Almost always occurs after 72 hours of administration
• NOT predicted by a history of rash or by skin tests