The utility of rapid Group A streptococcus molecular testing compared with throat culture for the diagnosis of Group A streptococcal pharyngitis in a high-incidence rheumatic fever population.

Background: Group A Streptococcus (GAS) causes significant morbidity and mortality in New Zealand and is responsible for invasive disease and immune sequelae including acute Rheumatic Fever (ARF). Early treatment of GAS pharyngitis reduces the risk of ARF. In settings with a high-burden of GAS disease, a rapid GAS pharyngitis diagnostic test with a strong negative predictive value is needed to enable prompt and accurate treatment. Methods: This prospective study compares the Xpert® Xpress Strep A molecular test (Cepheid) to throat culture and a second molecular method, the BioGX Group A Streptococcus-OSR for BD MAXTM for the diagnosis of GAS pharyngitis. Throat swabs were collected from the emergency department and wards of Middlemore Hospital, New Zealand. The BioGX Group A Streptococcus- OSR for BD MAXTM, contributes to the composite gold-standard: throat culture or both molecular methods positive. Basic demographic, clinical and laboratory data was collected. Results: 205/214 swabs were suitable for analysis. 28/205 (13.7%) were GAS culture positive, 45/205 (22%) Xpert® Xpress Strep A positive and 38/205 (18.5%) BioGX positive. Compared to culture, the sensitivity, specificity, positive and negative predictive values of the Xpert® Xpress Strep A molecular test were 100%, 90.4%, 62.2% and 100%, respectively. Compared to the composite gold-standard, the sensitivity, specificity, PPV and NPV and were 100%, 95.8%, 84.4% and 100% respectively. 17 samples were Xpert® Xpress positive but culture-negative; 6 of these 17 swabs represent true positives with evidence of recent GAS infection. Ten samples were culture negative but both Xpert® Xpress and BioGX positive. Conclusion: The Xpert® Xpress Strep A molecular test is highly sensitive with a strong negative predictive value and rapid turnaround time. It can be safely introduced as a first line test for throat swabs in a high-incidence ARF population.