Penicillin
A history of allergy to penicillin and a positive skin prick or intradermal test indicates IgE mediated hypersensitivity. The patient should not be administered penicillin (unless desensitized beforehand) and should wear a Medic Alert bracelet if the reaction was severe.
The absence of specific IgE to penicillin does not preclude the possibility of a previous allergic reaction as specific IgE may be lost over time. However, its absence indicates that a patient has essentially the same chance of reacting to the next course of penicillin as a patient without a history of penicillin allergy, i.e. 1-3% risk of a minor reaction, <0.1% risk of anaphylaxis.
Subsequent administration of penicillin may result in resensitisation (this risk is greater after parenteral rather than oral administration). Consequently it is recommended that penicillins are reserved for the treatment of infections for which they offer a clear clinical advantage over other antibiotics.
Cephalosporins
Testing with cephalosporins has much less predictive value than penicillins. A positive skin prick or intradermal test suggests IgE mediated hypersensitivity and cephalosporins should be considered contra-indicated for that patient. A negative test does not ensure that a hypersensitivity reaction will not occur on re-exposure to the drug. If indicated, a drug challenge is required to confirm the allergic status of a patient.
Cross-reactivity between penicillin and cephalosporins
The degree of cross-reactivity between penicillin and cephalosporins remains unresolved but is probably much less than is commonly perceived. Earlier studies suggested reaction rates of up to 10% when cephalosporins were given to penicillin allergic patients (the incidence of hypersensitivity reactions to cephalosporins in patients without a history of penicillin allergy is about 4%, with a very low rate of anaphylaxis). The rate of cross-reactivity is probably higher between penicillin and first generation cephalosporins. Positive penicillin skin prick tests do not predict the likelihood of allergic reactions to cephalosporins. Patients with both penicillin and cephalosporin allergies usually do not have cross-reacting antibodies, suggesting that the allergies are unrelated. A conservative policy would be to prescribe cephalosporins to penicillin allergic patients except when the reaction to penicillin was anaphylactic in nature.
References
1. Penicillin Allergy Test, Dr William Smith, Dr Karen Morwood, Royal Adelaide Hospital, Clinical Immunology and Allergy Unit Protocols, Revised March 2005.
2. Penicillin Allergenic Determinants (DAP). Kit insert revised Sep, 2006.This insert in turn refers to: Position paper: ?Diagnosis of Immediate Reactions to Beta ? Lactam Antibiotics? Torres MJ, Blanca M, Fernadez J, Romano A, Weck A, Aberer W, Brockow K, Pichler WJ, Demoli P, ENDA ; EAACI Interest Group on Drug Hypersensitivity. Allergy. 2003 Oct:58 (10);961-72.