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.