The vast majority of patients with a history of ...



Summary of Penicillin (PCN) allergy and recommended testing for practice

(originally developed in 2003)

Since 2003, nto much has changed except for the apparent increasing prevalence in adverse reactions to multiple medications with the classic one being related to beta-lactam antibiotcs, the “penicillins”. The vast majority of patients with a history of “penicillin allergy” can safely receive ß-lactam antibiotics. It is known that 90% of patients with a history of PCN allergy have negative skin tests. However, without testing, it is impossible to pick out those with true allergy for which a ß-lactam antibiotic could be fatal or at least life-threatening.

Immunological studies show a partial and variable cross-reactivity of cephalosporin and penicillin antibodies. Patients with positive skin tests to PCN are 7 times more likely to react to a cephalosporin. One large study of 350,000 adverse drug reactions, reported 12 fatal anaphylactic reactions to antimicrobials. 6/12 followed the first course of a cephalosporin and 4 or the 6 had a known allergy to PCN.

When the history is questionable or the future need for this category of drugs are predictable, it is advisable to engage drug testing with PCN before the need arises. The PCN testing can be completed in the office using Pre-pen, an in-house Minor Determinant Mix, Amoxicillin, and Penicillin-G. If skin testing is negative there is a 99% chance that the patient will not have a reaction to the drug. If skin tests are positive, there is a 50% chance that the patient will react to the drug. If negative, a provocative oral dosing of PCN or Amoxicillin is then conducted during the same visit. The drug is then continued for 5 days. If the challenge is negative, it is safe to use PCN in the future. If no prior reaction has occurred with a cephalosporin, then that group of drugs would also be considered safe. However, if the suspected drug allergy is to a cephalosporin, then following a negative PCN challenge (PCN testing should be conducted even if no history of PCN allergy), there should also be skin testing and provocative challenge to a cephalosporin that would be used in the future. Patients undergoing surgery are frequently placed on cefazolin as a prophylactic antimicrobial agent. Otherwise they are given vancomycin. Once it has been determined that PCN and/or cephalosporins can safely be administered, vancomycin should no longer be required.

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