Background: Although skin tests are usually employed to evaluate current penicillin allergy status, a negative result does not exclude hypersensitivity. There is a need for accurate in vitro tests to exclude hypersensitivity. A radioallergosorbent test (RAST) is a potentially good supplementary approach, but there is little information on the suitability of this method to diagnose penicillin hypersensitivity in subjects with a negative skin test to benzylpenicillin.
Methods: A total of 133 patients with a negative skin test to benzylpenicillin G (PG) and all of whom developed allergic reactions to PG were studied. RAST was used to detect eight kinds of specific IgE antibodies to penicillins in serum, which included four kinds of major and minor antigenic determinants to four penicillin drugs. The combination sites for the specific IgE antibodies were studied by RAST inhibition test.
Results: The rate of positive reactions for the specific IgE antibodies was 59.40% (79/133). Of the eight kinds of antigenic determinants, the positive rates for specific IgE against the major and minor determinants were 39.10% (52) and 42.86% (57) respectively. Of the four drugs, positive cases only to PG were 10 (7.5%), were significantly fewer than the cross-reacting positive cases (36) to PG (P < 0.01). In the RAST inhibition studies all drugs exhibited good inhibitory potencies, and in some instances the side-chain of the penicillins could induce specific responses with a variable degree of cross-reactivity among the different penicillins.
Conclusion: Radioallergosorbent test is a good complementary test in persons who are skin-test negative with PG, and the sensitivity of RAST increases with increasing specificity of IgE antibodies to be detected. 6-APA and the groups, making part of the different side-chains on penicillins, all contributed to the cross-reactivity.