Rapid diagnosis of group A streptococcal antigen extracted directly from swabs by an enzymatic procedure and used to detect pharyngitis.

Coagglutination after enzymatic digestion is a widely used, rapid procedure for serogrouping isolated colonies of beta-hemolytic streptococci. We tried to determine whether the same procedure could be used for the detection of group A streptococcal antigen directly from swabs used to take throat samples. This was achieved by incubating the swabs immersed in a small quantity of lytic extract obtained from cultures of the Maxted strain of Streptomyces griseus and testing the supernatant fluid by coagglutination. Of 538 throat swabs tested, blindly comparing the results of conventional cultures and rapid antigen detection, both tests were negative in 480 and both were positive in 49 swabs. In six cases, culture was positive and the rapid test was negative, but only one swab was from a patient with acute pharyngitis. In three cases, cultures were negative but the rapid test showed a strongly positive reaction. No special instructions were given to the physicians taking the samples. We conclude that this rapid antigen detection test, giving results in approximately 1 h, is an economic and reliable procedure for the detection of group A streptococcal antigen directly from throat samples.