Performance Evaluation of Serial SARS-CoV-2 Rapid Antigen Testing
To address high COVID-19 burden in U.S. nursing homes, rapid SARS-CoV-2 antigen tests have been widely distributed in those facilities. However, performance data are lacking, especially in asymptomatic people.
To evaluate the performance of SARS-CoV-2 antigen testing when used for facility-wide testing during a nursing home outbreak.
A prospective evaluation involving 3 facility-wide rounds of testing where paired respiratory specimens were collected to evaluate the performance of the BinaxNOW antigen test compared with virus culture and real-time reverse transcription polymerase chain reaction (RT-PCR). Early and late infection were defined using changes in RT-PCR cycle threshold values and prior test results.
A nursing home with an ongoing SARS-CoV-2 outbreak.
532 paired specimens collected from 234 available residents and staff.
Percentage of positive agreement (PPA) and percentage of negative agreement (PNA) for BinaxNOW compared with RT-PCR and virus culture.
BinaxNOW PPA with virus culture, used for detection of replication-competent virus, was 95%. However, the overall PPA of antigen testing with RT-PCR was 69%, and PNA was 98%. When only the first positive test result was analyzed for each participant, PPA of antigen testing with RT-PCR was 82% among 45 symptomatic people and 52% among 343 asymptomatic people. Compared with RT-PCR and virus culture, the BinaxNOW test performed well in early infection (86% and 95%, respectively) and poorly in late infection (51% and no recovered virus, respectively).
Accurate symptom ascertainment was challenging in nursing home residents; test performance may not be representative of testing done by nonlaboratory staff.
Despite lower positive agreement compared with RT-PCR, antigen test positivity had higher agreement with shedding of replication-competent virus. These results suggest that antigen testing could be a useful tool to rapidly identify contagious people at risk for transmitting SARS-CoV-2 during nascent outbreaks and help reduce COVID-19 burden in nursing homes.
Primary Funding Source:
As of 10 January 2021, in the United States, 1 022 297 nursing home residents and staff have tested positive for SARS-CoV-2, the virus that causes COVID-19, and 108 447 have died (1). Nursing home residents might be asymptomatic, have atypical symptoms, or be unable to verbalize their symptoms, making diagnosis using symptom-based screening alone inadequate (2, 3). Serial, facility-wide testing for SARS-CoV-2 can help identify cases in outbreak settings, allowing for rapid implementation of transmission-based precautions and infection prevention and control strategies (3, 4). Although real-time reverse transcription polymerase chain reaction (RT-PCR) testing performed in a laboratory has the highest sensitivity, its prolonged turnaround time can delay quarantine and isolation implementation (5, 6). Furthermore, RT-PCR can be a poor indicator for infectiousness because people might shed measurable amounts of viral RNA despite the absence of infectious virus (7–10). Conversely, the ability to culture virus from clinical specimens is a better indication of contagiousness than RT-PCR (11). Positive virus culture is most often detected within 10 days after onset or when viral loads are high (>7.0 log10 copies/mL) (12, 13).
Antigen tests are easy to use and produce results in minutes, facilitating rapid action, particularly during outbreaks in congregate settings (4, 14, 15). In 2020, the U.S. Food and Drug Administration granted emergency use authorization (EUA) to 11 rapid antigen tests. The U.S. Department of Health and Human Services sent 3 of these, including the Abbott BinaxNOW COVID-19 Ag Card, to nursing homes nationwide (16). According to the 3 products’ EUAs, among symptomatic people tested 5 to 7 days from symptom onset, the percentage of positive agreement (PPA) of antigen tests with RT-PCR is 84% to 99% and the percentage of negative agreement (PNA) remains close to 100% (16). However, antigen test performance in asymptomatic people and those with longer time to symptom onset than defined in the EUAs is not well characterized, with mixed reports on performance and concerns about false-positive results (16–19). Although mathematical models have suggested potential benefits from frequent, rapid-turnaround testing even with lower-PPA tests, limited data exist on antigen test performance in capturing early SARS-CoV-2 infections when people are most likely to be contagious (20–22).
On 7 October 2020, a 149-bed nursing home in Georgia identified its index COVID-19 case in a resident using the BinaxNOW antigen test, which prompted additional antigen testing in the facility. Despite attempts to implement mitigation measures, including cohorting, 43 residents and 5 staff had tested positive for SARS-CoV-2 by 21 October. The Centers for Disease Control and Prevention (CDC) worked with the Georgia Department of Public Health to evaluate the performance of the BinaxNOW antigen test compared with RT-PCR and virus culture. This report describes test characteristics of the BinaxNOW antigen test platform when used for symptomatic and asymptomatic people tested serially every 5 or 6 days during a nursing home outbreak.
Study Design and Data Sources
Between 22 October and 3 November 2020, serial, facility-wide testing of all residents and staff was done 3 times over a 13-day period during an ongoing SARS-CoV-2 outbreak. Specimens were collected from all available and assenting residents and staff present on days of testing, including people identified as SARS-CoV-2–positive before 21 October. During the first round of facility-wide testing, trained project personnel collected paired bilateral swabs from the anterior nares (AN) of residents for antigen testing and RT-PCR and, from nursing home staff, an AN swab for antigen testing and a nasopharyngeal swab from a single naris for RT-PCR. Because of patient intolerance, nasopharyngeal swabbing was discontinued during the second and third testing rounds and paired bilateral AN swabs were collected from both residents and staff (Appendix 2). All specimens were collected in accordance with CDC guidelines for specimen collection and handling (4). Trained laboratory scientists tested 1 AN swab onsite using the BinaxNOW COVID-19 Ag Cards per manufacturer instructions for use (23). The other was sent to the CDC for RT-PCR and virus culture reference testing.
The facility provided demographic characteristics and prior antigen testing results for residents and staff. During 7 to 21 October, the facility exclusively used BinaxNOW testing, and prior antigen positivity was defined as any positive result on a SARS-CoV-2 test during this time. At each visit, project personnel administered a standardized questionnaire assessing COVID-19–like symptoms (24). Able residents and staff self-reported symptoms at the time of testing. For residents who could not self-report, symptom information was obtained from nursing staff and electronic medical records and confirmed by residents, if possible. A symptomatic participant was defined as a resident or staff member who, at the time of collection, reported any new or worsening symptoms similar to those of COVID-19 (24) in the 14 days before that round of testing.
Participant specimens were tested for SARS-CoV-2 RNA by RT-PCR using the CDC Influenza SARS-CoV-2 (Flu SC2) Multiplex Assay (25) on the Applied Biosystems 7500 Fast Dx Real-Time PCR Instrument (Thermo Fisher Scientific). Nucleic acid was extracted by either the QIAGEN EZ1 or the Roche MagNA Pure 96 extraction platforms. Cycle threshold (Ct) values were reported for the SARS-CoV-2 viral nucleocapsid protein gene target. Values less than 40 indicated that a specimen was positive for SARS-CoV-2 RNA. Previous experience showed an inability to detect culture-positive virus in samples with a Ct greater than 34. Therefore, virus culture was attempted on RT-PCR–positive specimens with Ct values of 34 or less and RT-PCR–negative, antigen-positive specimens. Culture was done using Vero CCL-81 cells, as previously described (26). Cells showing cytopathic effect up to 8 days after culture inoculation were tested for the presence of SARS-CoV-2 by RT-PCR to confirm virus isolation and growth in culture (Appendix 2).
Specimens were categorized into stages of infection using prior test results and Ct values. Stages were defined as early (low or decreasing Ct values), late (increasing or sustained high Ct values), resolved (negative test result in a person with a prior positive result), or uninfected (consecutive negative results in specimens from a person with no prior positive result). Table 1gives full definitions.