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It seems we've finally put to bed the myth that 10% of penicillin-allergic patients will also react to cephalosporins. Dr. Campagna, et al. recently published a review article concluding that the true cross-reactivity is negligible except when side-chains are similar [PMID 21742459].
This topic was also the subject of a recent post on the Academic Life in EM blog (http://academiclifeinem.blogspot.com/2012/08/busting-myth-10-cephalosporin.html).
But what about the reverse question? Can I give a penicillin to a cephalosporin-allergic patient?
Dr. Romano's group tested 98 patients with skin-test postitive cepahlosprin allergy (mostly IgE -mediated anaphylaxis). Patients were then skin tested for penicillin allergy. Those testing negative were challenged with a penicillin.
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25% of patients reacted to the penicillin
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Similar side-chain was a strong predictor of cross-reactivity
A Letter to the Editor response to this study pointed out that the authors used a smaller-than-standard size threshold for a positive response to the penicllin AND used a higher-than-standard dose of amoxicillin for testing. In light of this, the rate of subjects with cephalosporin allergy who do not have a history of penicillin allergy but with true IgE-mediated allergy to penicillin might be much closer to 5%.
Bottom line: The cross-reactivity of penicillins in cephalosporin-allergic patients is somewhere between 5-25%.
References
Romano A, et al. IgE-mediated hypersensitivity to cephalosporings: cross-reactivity and tolerability of penicillins, monobactams, and carbapenems. J Allergy Clin Immunol 2010;126(5):994-9.
Macy E. Penicillin allergy might not be very common in subjects with cephalosporin allergy. J Allergy Clin Immunol 2011;127(6):1638-9.
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