Benzylpenicilloyl Polylysine is a skin-testing reagent which used to detect immunoglobulin E antibodies in people with a history of penicillin allergy. The quantitation of in vitro IgE antibodies to the benzylpenicilloyl determinant is a useful tool for evaluating allergic subjects. A penicillin skin test predicts only the presence of IgE antibodies for the major or minor penicillin determinants at the time of application and does not predict the future development of IgE-mediated reactions during subsequent courses of penicillin. Benzylpenicilloyl polylysine reacts specifically with penicilloyl skin sensitizing antibodies (reagins) to produce immediate wheal and flare reactions which may reflect increased risk of allergic reactions to subsequent penicillin therapy. The use of benzylpenicilloyl polylysine can detect between 75-90% of all positive reactions to penicillin. Benzylpenicilloyl polylysine was FDA approved in 2009 for the assessment of sensitization to penicillin (benzylpenicillin or penicillin G) in those patients suspected of having a clinical hypersensitivity to penicillin.