Abstract
BACKGROUND: Current statistics show that approximately 10% of patients claim to be allergic to penicillin yet only 10% of these have demonstrable allergy. The most appropriate and cost-effective antibiotics are sometimes withheld on the basis of patient history of drug allergy. OBJECTIVE: Investigation of IgE hypersensitivity and delayed hypersensitivity in patients with a history of penicillin allergy to a teaching hospital allergy clinic. METHODS: Patients underwent skin prick and intradermal testing (IDT) with major and minor penicillin determinants. Those with negative skin tests were administered a three-day oral challenge. Demographic and clinical details about the reactions were noted. RESULTS: One hundred twenty eight patients underwent testing, of these, one hundred and ten had self-reported histories of penicillin allergy and eighteen were referred because of other antibiotic allergies. Seventeen patients with self-reported penicillin allergy had either positive skin tests or oral challenge results, corresponding to 15% of patients having proven allergy. None reacted on skin prick testing, four reacted to IDT, thirteen reacted to oral challenge (five immediate and eight delayed). Analysis of clinical histories showed that patients with a well-defined history of allergy and a history of anaphylaxis were more likely to have a positive test compared to patients with vague histories. Skin testing proved to be less sensitive than oral challenge. CONCLUSION: A minority of patients presenting with a history of penicillin allergy have evidence of immune-mediated hypersensitivity (17/110, 15%) in this study. Of these, eight out of seventeen (47%) had delayed reactions, demonstrating the usefulness and discriminating power of objective testing, which must include three-day oral challenge. Discriminating factors for immune-mediated allergy from patient history were a clear description of the original reaction and a history of anaphylaxis. Negative allergy testing enables the use of penicillin as first-line treatment when necessary and this can significantly reduce costs of antibiotics.