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Why is penicillin allergy so prevalent?

Q - Why is penicillin allergy so common?

A - It is said that penicillin and its derivatives account for 75 per cent of all severe allergic reactions to drugs. Atopy is not a risk factor. In general practice many patients report 'allergy' to penicillin. Often this is on the basis of a maculopapular rash, which is unlikely to be due to IgE-mediated allergy.

Reports of anaphylaxis and angio-oedema are much more likely to be markers of true allergy. In many cases though, an urticarial rash is as likely to have been triggered by the infection as the antibiotic for which it has been prescribed. The description and the timing are all-important.

Reactions to penicillin are prevalent because it is probably the most widely prescribed antibiotic, and the penicillin molecule is uniquely reactive.

A similar molecular structure is found in cephalosporins: 10 per cent of penicillin allergic patients will react to cephalosporins. Cross-reactivity will also occur with the newer monobactams and carbapenems.

Other metabolites can also trigger acute allergic reactions ­ known as the minor determinants. Blood tests for specific IgE to penicillin, which only detect IgE to the major determinant, will therefore only identify a proportion of penicillin-allergic patients. Skin-prick testing with both the major and minor determinants is therefore required and this should be carried out in centres experienced in the diagnosis of drug allergy. Skin testing, however, is only of value in identifying IgE-mediated allergy, it will not identify the immunological reactions responsible for penicillin-triggered haemolysis, contact dermatitis or erythema multiforme. The pick-up rate in patients with maculopapular rashes is low.

Penicillin hypersensitivity declines with time after a reaction, so skin-prick testing also diminishes with time. With a clear history of an immediate reaction, 93 per cent are skin test positive within seven to 12 months of the reaction, while only 22 per cent are positive 10 years or more after a reaction.

The pragmatic approach is to use alternatives where possible and test only those with a definite need for penicillin.

Dr Gavin Spickett is a consultant clinical immunologist at

Royal Victoria Infirmary, Newcastle

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