ABSTRACT
Background South Africa was the African country most severely affected by the SARS-CoV-2 pandemic during 2020, experiencing 2 waves of infection. During the first wave, diagnostics were largely based on reverse transcription-linked PCR (RT-PCR). The Abbott PanBio antigen test was deployed during the 2nd wave which was driven by emergence of the 501Y.v2 variant. At the time of evaluation in mid-November 2020, 501Y.v2 was the dominant circulating virus in Nelson Mandela Bay, in the Eastern Cape Province. Methods A prospective diagnostic evaluation study was undertaken, during a period of high community transmission, to evaluate the field performance of the PanBio antigen RTD. Testing was conducted at mobile community testing centres on 677 ambulant patients seeking SARS-CoV-2 testing. RT-PCR was performed on the original naso-pharyngeal antigen swabs to evaluate test performance. Results Of 146 RT-PCR positive individuals, 101 were RTD positive in the clinic. The antigen RTD had an overall sensitivity of 69.2% (95%CI 61.4, 75.8) and specificity of 99.0% (95%CI 98.8, 99.3) in this clinical context. Sensitivity was strongly dependent on the amount of virus in clinical samples, as reflected by the PCR cycle threshold (CT) value, with 100% detection in samples where the CT was <20, 96% with CT between 20-25, 89% with CT between 26-30 and 64% when CT was 31-35. Conclusions The assay reliably detected 501Y.v2 infections in ambulatory ill patients. Assay sensitivity was >90% in patients with high viral loads who are expected to be most infectious. Negative and positive predictive values were also >90%.