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Studies of fatal anaphylaxis in the UK have shown that widely prescribed β-lactam antibiotics are responsible for 26% of fatal drug induced anaphylaxis and 11% of all cases of fatal anaphylaxis.1 A history of allergy to a penicillin or cephalosporin, common in both community and hospitalised patients, therefore cannot be ignored and must always be recorded. However, there are numerous reports that four in five individuals who believe they are allergic to penicillins will in fact tolerate penicillin use, so for these patients the constraints on antibiotic choice, potentially resulting in less effective and more expensive treatment, are unnecessary. Drug allergy work-up is often cost effective, with a favourable risk:benefit ratio, but controversies have arisen recently regarding both the reagents and the protocols considered best practice for penicillin skin testing.
DECIDING WHO TO TEST
Several tests are available to detect penicillin specific IgE antibodies, the presence of which indicates a substantially increased risk of immediate allergic reaction with penicillin use. Before testing a history-positive patient it is important to decide whether such investigations are relevant—for some patients investigation is not appropriate. For example, the history of particular life threatening adverse reactions, including Stevens–Johnson syndrome, toxic epidermal necrolysis, vasculitis, drug induced autoimmune cytopenias, hepatitis and severe organ involvement, is considered an absolute contraindication to future use of the relevant antibiotic. These reactions are not IgE mediated and testing for specific IgE antibodies is not appropriate. Nor is further investigation needed if a detailed history reveals that the reaction was a common adverse effect, such as diarrhoea, incorrectly recorded as allergy. When appropriate, skin testing and serum testing can be …