Evaluation of Copan swabs with liquid transport media for use in the Gen-Probe Group A Strep Direct Test

The Gen-Probe Group A Streptococcus Direct Test (GASDT), which detects the presence or absence of group A streptococci directly from pharyngeal specimens, utilizes a specific relative light unit (RLU) cutoff of 4,500 to differentiate between positive and negative test results. In response to a report by a manufacturer that the background RLU values for the Copan rayon swabs with liquid media were higher than the RLU values typically observed with Culturette swabs, we tested multiple lots of Copan rayon swabs with liquid media and determined that the swabs are unacceptable for routine use in the GASDT. The high background RLU values for the Copan rayon swabs appear to be a direct result of the gamma irradiation used to sterilize the swabs. We also performed a comparative clinical evaluation of Copan Dacron swabs with liquid media and Culturette swabs for use in the GASDT. Overall, there was 97.5% agreement between the results obtained with the Copan Dacron swabs and those obtained with the Culturette swabs. Compared to Culturette swabs, the Copan Dacron swabs had a sensitivity and a specificity of 97 and 98%, respectively. Copan Dacron swabs with liquid media are an acceptable alternative to the swabs currently qualified for use with the GASDT, but Copan rayon swabs with liquid transport media should not be used in the GASDT.