Comparison of Cepheid Xpert Xpress SARS-CoV-2 Assay with Standard RTPCR Test for Detection of COVID-19 Infection: A Retrospective Cohort Study

Introduction: Real time Reverse Transcription-Polymerase Chain Reaction (RT-PCR) test, the gold standard test for Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) detection, is a tedious process and requires proficient workforce. Accurate and fast test results may permit more efficient use of protective and isolation resources and allow rapid therapeutic interventions.

Aim: To evaluate the analytical performance characteristics of the Cepheid Xpert Xpress SARS-CoV-2 test, a rapid, automated molecular test for SARS-CoV-2 with gold standard RT-PCR test.

Materials and Methods: This retrospective cohort study was conducted in Virus Research and Diagnostic Laboratory (VRDL) in Department of Microbiology at GGS Medical College, Faridkot, Punjab, India, from January to June 2021. A total of 100 nasopharyngeal samples, collected from clinically suspected Coronavirus Diseae-2019 (COVID-19) cases admitted at GGSMC during 1st January-30th June 2021 were tested both by Xpert assay and RT-PCR test simultaneously, taking RT-PCR as the gold standard test. The data was analysed by MedCalc® statistical software version 19.6.4., and sensitivity, specificity, predictive values, likelihood ratios and the agreement between the two tests were calculated.

Results: The mean age of the study participants was 46 years. Of these, 55 were males and 45 were females. The overall sample sensitivity and specificity of the Xpert assay were both 100% and there was perfect agreement across specimens, if authors, set a cut-off Cycle threshold value (Ct value) at 40 cycles for Xpert. Of 100 samples, 32 were positive for SARS-CoV-2 by either of the tests and 68 were negative. Xpert assay could detect 100% positive cases and RT-PCR test could detect 84.37% positive cases. Out of the 32 samples which were positive by Xpert assay, 5 (15.62%) samples had a Ct value greater than 40.

Conclusion: The Xpert assay found to be useful as a point-of-care test in acute scenario, where rapid and authentic diagnosis is essential, but do not have expertise and infrastructure to perform RT-PCR.