False Positive Strep a Antigen Test

To The Editors: We read with interest Rubin and Mirkin’s article. 1 We recently completed a study comparing two rapid group A Streptococcus kits, the Abbott TestPack + Plus Strep A (Abbott Laboratories, Abbott Park, IL) and BioStar Acceava StrepA (BioStar, Inc., Boulder, CO). 2 In this study specimens from all patients were cultured for Streptococcus pyogenes on standard sheep blood agar. Cultures from specimens that were positive by either rapid method but did not reveal typical beta-hemolytic group A streptococci were further scrutinized for the presence of S. pyogenes by examining hemolysis after anaerobic incubation and performing a sweep of the first quadrant and latex agglutination for the presence of Lancefield group A antigen with the Strep Rapid Latex test (Murex Diagnostic, Inc., Norcross, GA). If latex agglutination occurred, then isolation plates were made and individual isolates were tested for the presence of group A antigen by latex agglutination. In so doing we encountered two isolates that were alpha-hemolytic and repeatedly reactive in the group A latex agglutination test. These isolates were also pyroglutamyl aminopeptidase-positive and were confirmed as S. pyogenes by the Gen-Probe Group A Streptococcus Direct Test (Gen-Probe Inc., San Diego, CA). We agree with Rubin and Mirkin that given the high specificity of the rapid screens for group A Streptococcus, atypical group A streptococci should be excluded before a rapid group A Streptococcus screen should be considered a false positive when the rapid screen is compared with culture. In our study we identified 2 alpha-hemolytic S. pyogenes isolates in 106 culture-positive specimens. These isolates were from 2 patients who presented with acute pharyngitis to the same clinic. Microbiologists and clinicians should be aware of alpha-hemolytic varieties of S. pyogenes in addition to the nonhemolytic variety described. Although alpha-hemolytic varieties of S. pyogenes are infrequently encountered, these could potentially be dismissed as viridans streptococci or Streptococcus pneumoniae in cultures from the upper airways, and the diagnosis of acute streptococcal pharyngitis, with its potential sequelae, would be missed. Suzanne Schroeder M., T., (ASCP) Gary W. Procop M.D., M., S.