Evaluation of Xpert® Xpress Strep A Test for detection of Group A streptococcus in throat swabs

Streptococcus pyogenes (Group A β -hemolytic Streptococcus) infections in humans range from mild pharyngitis to life-threatening infections such as septicaemia and necrotizing fasciitis. We assessed the diagnostic accuracy of the Xpert® Xpress Strep A Assay, a new automated real-time polymerase chain reaction (PCR) that can detect this pathogen.

Methods

We evaluated the diagnostic accuracy (sensitivity, specificity PPV and NPV) of Xpert® Xpress Strep A Test (Cepheid, Maurens-Scopont, France) for the detection of Group A Streptococcus in throat swabs using Copan virus transport medium at Ninewells Hospital & Medical School (Dundee, Scotland, UK). The performance of Xpert t® Xpress Strep A Assay was compared with routine culture (gold standard method).

A total of 323 clinical throat swab samples were tested. The sensitivity of Xpert ® Xpress Strep A Assay was found to be 95% (95% CI, 75.13% -99.87); specificity 96.5% (95% CI 93.83% -98.36%);PPV 65.5% (95% CI 50.61% -77.89%); and NPV 99%(95% CI 97.68% -99.95%). Overall, there were eleven discrepant results, of these 10 were positive by PCR and not confirmed by culture and one sample was recorded positive by culture and negative by PCR. The turnaround time (TAT) was 24 minutes for negative samples and 18 minutes for positive samples.

The Xpert® Xpress Strep A Assay for detection of Streptococcus pyogenes in throat swabs had high sensitivity, specificity and rapid TAT. Use of this technology as a point-of-care test could be explored further to assess its impact on antimicrobial prescribing and infection prevention and control measures.