Evaluation of a rapid latex agglutination test (Directigen®) for the direct detection of group A streptococci from throat swabs

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Abstract
Summary A rapid group A beta-hemolytic streptococci (GAS) antigen detection test using latex agglutination (Directigen®, Becton Dickinson) was compared with a conventional culture method for the direct detection of GAS from throat swabs. One throat specimen was collected from each of 229 patients. After standard inoculation onto sheep blood agar plates, all swabs were tested for GAS antigen. Of the 229 specimens tested, 46 (20%) were GAS-positive by culture. Direct latex agglutination agreed with culture in 222 of them (97%). The sensitivity of the test was 93% (43/46), the specificity 98% (179/183), the positive predictive value 91%, and the negative predictive value 98%. The detection of GAS antigen by coagglutination (Phadebact®, Pharmacia) was also carried out. 81% (35/43) of the cultures and Directigen®-positive specimens gave a positive coagglutination for GAS. 100% (179/179) of the cultures and Directigen®-negative specimens were also negative by the coagglutination test. We conclude that the Directigen® rapid latex agglutination test for the direct detection of GAS from throat swabs compares favorably with culture and has the advantage of providing same-day results.