Rapid identification of group A streptococci by the Strep-A-Fluor system

Group A streptococci can be rapidly identified by the Strep-A-Fluor system (Bio-Spec Inc., Dublin, CA), which uses a specific aminopeptidase to hydrolize L-pyrrolidonyl-beta-naphthylamide. In pure subcultured colonies and in isolated colonies from primary culture plates, the Strep-A-Fluor system had 100% sensitivity and 100% specificity compared with the results of Streptex latex agglutination (Wellcome Research Laboratories, Research Triangle Park, NC). However, in mixed culture from primary culture plates, the sensitivity and specificity were 73% and 95%, respectively. Furthermore, in a routine laboratory setting, the sensitivity was only 91% both from pure subcultured colonies and from isolated colonies from primary culture plates. Contamination with organism other than group A streptococci that are capable of hydrolyzing L-pyrrolidonyl-beta-naphthylamide may explain the low specificity in mixed culture. Deterioration of the buffer reagent due to prolonged use may account for the low sensitivity in a routine laboratory setting.