Penicillin Peril

I am often confronted with the caesarean section patient with a “penicillin allergy” who then warrants a prophylactic dose of antibiotic. Often the “allergy” were gastrointestinal symptoms or a rash after an oral administration. These symptoms could either be related to the viral illness at the time that did not need an antibiotic in the first place or they could indeed be a minor IgE mediated hypersensitivity which has sensitised the patient to be at risk of a significant response to a subsequent exposure. Sometimes the allergy is a vague story from childhood and only rarely is it a well-documented anaphylaxis after an identified antibiotic.

The conundrum is: if and what to give for caesarean prophylaxis? The question can be even more pressing for the labouring parturient with gram positive streptococcus. Cross reactivities between the classes of betalactams mean the risk of anaphyaxis remains if a different drug is used.

Skin testing has been evaluated an is readily used in many centres for this situation. For example, Gadde et al from Johns Hopkins (JAMA. 1993 Nov 24) used skin testing on 5000 patients in a Baltimore STD clinic a found a low rate (7%) of positive tests even with those of a so-called allergy. In those without a reported “allergy” a negative skin test was 99.5% predictive of no response to therapeutic penicillin; in those with a reported “allergy” a negative skin test was 97.1% predictive of no response and the other 2.9% only had minor systemic reactions. Macy in San Diego (Ann Allergy Asthma Immunol. 2006 Aug) and other studies return similar reassurance on the value of skin testing though caution should still be taken with those with a strong history but negative skin test as a significant reaction may still rarely occur.

Most papers on the matter are published by academic allergy departments. These departments use highly structured and controlled allergy tests. Often the escalate the challenge with a topical patch, the a prick then a intradermal test. How do we manage the patient in theatre already under the drapes or in the chaos of the labour ward?

I suggest that we accept that we can only test for the drug to be used: it is not the time to test for the spectrum of betalactams. For penicillin, skin testing with an intradermal injection of 0.1 ml of 10000 IU/ml (a vial in 100 ml) can be done on the volar forearm. If a wheal of > 3mm has not developed after 15 min give 1 ml of this concentration of penicillin intravenously. It seems an IV test dose should be 1/100 of the therapeutic dose. Wait 5 minutes and slowly give the therapeutic dose.

For cephalosporin, I think 10 mg/ml is an appropriate skin test dose (the vial in 100 ml) followed by a 10 mg test dose.