Background

Approximately 10% of the patient population will report an allergic reaction to a penicillin class antibiotic. Of those who report a history of penicillin allergy, up to 90% are able to tolerate penicillins when challenged.

The reasons for the discrepancy between reported and actual penicillin allergy may be due to:

• Waning of penicillin specific IgE antibodies over time. It has been reported that up to 80% of patients may lose their IgE-mediated sensitivity to penicillins after 10 years of the initial reaction.

• A cutaneous reaction that was initially thought to be an allergy, but was actually a result of the underlying viral or bacterial infection, or an interaction between the infectious agent and the antibiotic given at the time.

• A drug side effect mislabeled as an allergic reaction.

The rate of penicillin-induced anaphylaxis is rare and is reported to be less than 1% in the literature.

In a large study evaluating the incidence of penicillin hypersensitivity (IgE mediated), it was found that in patients with a valid history of penicillin allergy, 21% had a positive skin test, while in patients with a questionable or invalid history, 10% and 4% had a positive skin test, respectively. Skin testing with both penicilloyl-poly-lysine (major determinant) and penicillin G (minor determinant) can identify up to 95% of patients with positive reactions. In the case of a negative skin test, 98% of patients will tolerate penicillin without any serious reaction. Penicillin skin testing or a graded challenge should be considered based on the patient’s allergy history.

Despite the limitations of the available literature, the reported rate of allergic reaction with cephalosporins in a patient with a history of penicillin allergy is less than 1%. Furthermore, the rate of reaction to cephalosporin in patients with a positive penicillin skin test is approximately 2%. Therefore in patients with a history of non-severe penicillin reaction, the administration of a cephalosporin (that does not share a similar side chain as the culprit penicillin) can be safely given.