Performance of Routine Rapid Antigen Diagnostic Testing and Bacterial Culture Compared with PCR Testing for the Detection of Group A Strep

BACKGROUND: Rapid antigen detection tests (RADT) and bacterial culture are the current standard of care for diagnosing Group A Strep in pediatric patients. Polymerase Chain Reaction (PCR) tests offer improved turn-around-times at the point-of-care (POC) or in the laboratory. PCR has demonstrated improved sensitivity over reference culture in previous studies. METHODS: The performance of the QuickVue Strep A test (RADT) and bacterial culture for detection of group A Strep was evaluated during the fall and winter seasons of 2016/17 at a pediatric primary care clinic. Concordant PCR results from the cobas(®) Liat(®) Strep A test (a POC PCR assay (POC PCR)), Solana GAS Assay (a lab based PCR assay (Lab PCR)) were used as the reference method. Two hundred and sixty-eight throat samples from children < 18 years of age were prospectively collected. RADT and POC PCR were conducted in the physician office, culture was conducted in the laboratory and Lab PCR was conducted on banked specimens. Final performance analysis of RADT, POC PCR, culture and LAB PCR included 246 patients. RESULTS: The prevalence of Strep A in this population was 40.2% (99/246). RADT demonstrated sensitivity of 88.9% (88/99) and specificity of 89.8% (132/147) compared with PCR. Of 11 RADT false-negative samples 2 were positive by culture. The 15 RADT false-positives were all negative by culture. Culture demonstrated a sensitivity of 77.8% (77/99) and specificity of 100% (147/147) compared with PCR with a median turn-around-time of 2 days. Of 22 false-negative culture results, 13 were RADT positive. Twenty-two subjects were excluded from the analysis due to discordant PCR results. A statistically significant relationship was found between Ct values for POC PCR positive samples and discordant results. The average Ct value of PCR and culture concordant positive results was 21.5, PCR and culture discordant results 27.6 and PCR discordant results 30.6. CONCLUSION: In this population false-positive RADT results were higher than expected and most false-negative RADT results would not be identified through routine diagnostic culture. Following current guidelines, these results would likely result in miss-diagnosis. PCR can offer simplification of testing and provide sensitive and specific results at the point-of-care. DISCLOSURES: U. Cowen, Roche Molecular Systems: Employee, Salary; S. Tang, Roche Molecular Systems: Employee, Salary; D. Duncan, Roche Molecular Systems: Employee, Salary; J. Sickler, Roche Molecular Systems: Employee, Salary