Prospective Evaluation of Xpert® Xpress Strep A Automated PCR Assay vs. Solana® Group A Streptococcal Nucleic Acid Amplification Testing vs. Conventional Throat Culture

Ferrieri, Patricia; Thonen-Kerr, Elizabeth; Nelson, K.; Arbefeville, Sophie

In our laboratory, the negative rapid group A streptococcal (GAS) antigen assays are backed up by the Solana® GAS Assay by Quidel instead of a Group A streptococcal throat culture. Another FDA cleared RT-PCR assay is the Xpert® Xpress Strep A, which detects Streptococcus pyogenes DNA, and is performed on the Cepheid GeneXpert instrument. Three hundred seventy-five positive and negative specimens were randomly selected from 5489 throat specimens that had been tested by the Solana® GAS Assay during January 2018 and were tested with the Xpress Strep A assay. A throat culture was also set up (sheep blood agar at 35 °C in 5% CO2). All beta-hemolytic streptococci were purified and identified by MALDI-TOF mass spectrometry. Of the 375 samples, 185 were positive by Solana® GAS Assay, and 187 were positive by the Xpress Strep A. The total agreement between the Solana® GAS Assay and the Xpert® Xpress Strep A was 99.5%. The agreement of the Xpert® Xpress Strep A assay with culture was 90.1%. The sensitivity and specificity for Xpress Strep A versus culture were 100% and 83.5%, respectively. The Xpert® Xpress Strep A assay’s performance was equivalent to the Solana® GAS Assay, and was highly sensitive. The lower specificity was likely due to the Xpress Strep A assay having higher sensitivity as compared to throat culture.