COVID-19 Antigen Testing: Better than We Know?

Background: Antigen testing (AGT) for SARS-CoV-2 is generally considered to be less sensitive than the standard reference method – RT-PCR. It has been suggested that a significant part of patients with positive RT-PCR “missed” by AGT might be free of viable virus and, thus, non-infectious. However, to the best of our knowledge, no head-to-head comparison of this has been performed so far.Methods: In a screening setting for asymptomatic or mildly symptomatic patients, 496 patients were tested using RT-PCR as well as a single AGT. Where the results differed, virus viability was evaluated by culture on CV-1 cells. Screening test parameters were calculated with RT-PCR and RT-PCR corrected on viability as reference standards.Findings: The sensitivity of the used AGT related to the RT-PCR only was mere 76·2%. However, 36 out of 39 patients “missed” by AGT contained no viable virus. After correction on that, the sensitivity grew to 97·7% and, more importantly for disease control purposes, the negative predictive value reached 99·2%.Interpretation: We propose that viability testing should be always performed when evaluating a new antigen test. Our results also indicate that a well-chosen and validated antigen test provides excellent results in the identification of patients who are shedding viable virus (although some caveats still remain) in the screening setting of asymptomatic or mildly symptomatic individuals.Funding Statement: This research was internally funded by the Hospital Karvina-Raj and the Public Health Institute Ostrava. Antigen tests were provided by the Ministry of Health of the Czech Republic.Declaration of Interests: All Authors declare that they have no conflict of interest regarding the research presented in this paper.Ethics Approval Statement: The study was approved by the local Ethics Committee, No. NsPKar/19956/2020/SEK.