ß-lactam allergy


  • 10% of patients report a penicillin allergy but less than 1% are truly allergic.

    • 0.01-0.05% of patients treated with penicillin experience anaphylaxis with a fatality rate of 0.0015-0.02%.

  • 0.0001-0.1% of patients treated with a cephalosporin experience anaphylaxis.

History of Penicillin Allergy

  • It is very important to determine:

    • the nature of the patient's reaction, in order to differentiate between allergic and other adverse reactions (e.g. diarrhea, nausea, vomiting, headache), and

    • the onset of the allergic reaction, which will help to classify the reaction (see Table). This will help determine whether β-lactam antibiotics can be used. Non-β-lactam alternatives are available for most indications however they may be less effective, more toxic, more broad spectrum, more expensive, more likely to result in colonization or infection with MRSA/VRE, and could lead to increased hospital length of stay, increased re-admission and Clostridioides (Clostridium) difficile infections.

  • Mild rash due to aminopenicillins (ampicillin, amoxicillin, pivampicillin) is often caused by a drug-viral interaction, e.g. mononucleosis (Epstein-Barr virus), and is not IgE-mediated.

  • 50% and 80% of penicillin allergic patients lose their sensitivity to penicillin after 5 and 10 years, respectively.

    • Positive penicillin skin tests decrease by ~10% per year.

    • ~80-90% of patients with a history of penicillin allergy may no longer react to penicillin.