Detection of group A streptococcal antigen from throat swabs by use of a latex agglutination test kit in general practice.

In 11 general practice offices, with a total of 29 general practitioners, throat swabs from 468 patients with acute pharyngotonsillitis were assayed for group A streptococci with a commercial antigen detection test kit (Culturette Brand Ten-Minute Group A Strep ID; Marion Scientific, Div. of Marion Laboratories, Inc., Kansas City, MO 64114, USA). Compared to aerobic cultures carried out in our laboratory, the sensitivity of the antigen detection test was 73%; the specificity, 98%; the positive predictive value, 96%; the negative predictive value, 84%; and the overall agreement with laboratory findings, 88%. The prevalence of group A streptococci was 42%. Tests performed by assistants were significantly less sensitive (52%) than tests carried out by the physicians themselves (77%). The sensitivity of office cultures was 83% for blood agar plates supplemented with bacitracin discs, and 77% for Streptocult. We conclude that antigen detection tests should not presently be substituted for office cultures in the diagnosis of group A streptococcal throat infections.