Performance Characteristics of a Rapid Severe Acute Respiratory Syndrome Coronavirus 2 Antigen Detection Assay at a Public Plaza Testing Site in San Francisco

Abstract

We evaluated the performance of the Abbott BinaxNOW rapid antigen test for coronavirus disease 2019 (Binax-CoV2) to detect virus among persons, regardless of symptoms, at a public plaza site of ongoing community transmission. Titration with cultured severe acute respiratory syndrome coronavirus 2 yielded a human observable threshold between 1.6 × 104-4.3 × 104 viral RNA copies (cycle threshold [Ct], 30.3-28.8). Among 878 subjects tested, 3% (26 of 878) were positive by reverse-transcription polymerase chain reaction, of whom 15 of 26 had a Ct <30, indicating high viral load; of these, 40% (6 of 15) were asymptomatic. Using this Ct threshold (<30) for Binax-CoV2 evaluation, the sensitivity of Binax-CoV2 was 93.3% (95% confidence interval, 68.1%-99.8%) (14 of 15) and the specificity was 99.9% (99.4%-99.9%) (855 of 856).

Keywords: COVID-19; Point of Care testing; Rapid Antigen Test; SARS-CoV-2.

Figure 1.

Titration of in vitro grown severe acute respiratory syndrome coronavirus 2 and detection with Binax-CoV2 assay. Top, Normalized Binax-CoV2 sample band intensity (blue-green average) for cards loaded with a known amount of virus. Error bars represent standard deviation of sample band intensity of technical replicates. Reverse-transcription polymerase chain reaction (RT-PCR) testing was performed at the CLIAHUB consortium [10]. Corresponding RT-PCR cycle threshold (Ct) values (average of N and E gene probes) are shown in black, and the corresponding RNA copy numbers in blue. Note that Ct and genome copy number correlation varies by RT-PCR platform. Bottom, Representative card images from each data point. Abbreviation: PFUs, plaque-forming units.

Figure 2.

Comparison of Binax-CoV2 test with quantitative reverse-transcription polymerase chain reaction (RT-PCR) test. A, Average viral cycle threshold (Ct) values from all 26 RT-PCR–positive individuals from the community study, plotted in ascending order. Blue circles indicate Binax-CoV2–positive samples; yellow squares, Binax-CoV2–negative samples. Open symbols represent individuals who were asymptomatic on the day of the test and filled symbols, those who reported symptoms on that day. B, Normalized sample band signal from retrospective image analysis of Binax-CoV2 cards was plotted as a function of Ct value for all available scanner images (19 of 26 RT-PCR–positive samples and a random subset of RT-PCR–negative samples). Binax-CoV2 true-positives are shown in blue and labeled TP; false-negatives, shown in yellow and labeled FN; and true-negatives, shown in red and labeled TN. C, Corresponding Binax-CoV2 card images from the data in B.