Analytical Performance of the Point-of-care BIOSYNEX COVID-19 Ag BSS for the Detection of SARS‐CoV‐2 Nucleocapsid Protein in Nasopharyngeal Swabs: A Prospective Field Evaluation During the COVID-19 Third Wave in France

Abstract Background: The accuracy and reliability of rapid diagnostic tests are critical for monitoring and diagnosing SARS-CoV-2 infection in the general population. This study aimed to evaluate the analytical performance of the BIOSYNEX COVID-19 Ag BSS (Biosynex Swiss SA, Fribourg, Switzerland) antigen rapid diagnostic test (Ag-RDT), which targets the SARS-CoV-2 N-nucleocapsid protein for the diagnosis of COVID-19. The Ag-RDT was compared with a real-time RT-PCR (rtRT-PCR) gold standard for performance measurement.Methods: Two nasopharyngeal flocked swabs were prospectively collected simultaneously in March and April 2021 from 967 individuals aged ≥18 years tested for SARS-CoV-2 in two private laboratories, Paris, France.Results: Overall, the Ag-RDT demonstrated high sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 81.8%, 99.6%, 96.6%, and 97.5%, respectively, as well as high or near-perfect agreement (97.0%), reliability was assessed using Cohen’s κ-coefficient (0.87), and accuracy was evaluated using Youden index (J) (81.6%) in detecting SARS-CoV-2. The analytical performance of the Ag-RDT remained high when there was significant viral shedding (i.e., N gene Ct values ≤ 33 on reference RT-PCR). The sensitivity was only 55.2% in case of low or very low viral excretion (Ct> 33).Conclusions: The BIOSYNEX COVID-19 Ag BSS Ag-RDT is a promising, potentially simple diagnostic tool, especially in symptomatic COVID -19 or proven infectiousness.