By Joseph F. John, Jr., MD, FACP, FIDSA, FSHEA

Clinical Professor of Medicine and Microbiology, Medical University of South Carolina and Lowcountry Infectious Diseases, Charleston

SYNOPSIS: Rapid antigen testing was accurate in detecting SARS-CoV-2 antigen when compared to polymerase chain reaction.

SOURCE: McKay SL, Tobolowsky FA, Moritz ED, et al. Performance evaluation of serial SARS-CoV-2 rapid antigen testing during a nursing home outbreak. Ann Intern Med 2021;174:945-951.

The objective of this study was to determine the efficacy and role of rapid antigen testing for SARS-CoV-2 infection during a nursing home outbreak. The period was October and November 2020, with the first case identified on Oct. 7, 2020. During just a 13-day period, all staff and patients were tested up to three times if they were at the facility on the day of testing. The Abbott BinaxNOW COVID-19 Ag Card was used to perform antigen testing. Along with rapid antigen testing of samples from each nare, a reverse transcription polymerase chain reaction (RT-PCR) for COVID-19 analysis also was conducted to determine the relative usefulness of a rapid antigen test. Results showed 107 staff and 127 residents took part in at least one round of testing, and there were 234 total participants. Among residents, the median age was 75 years; 43% were female and 60% were Black. During the three testing periods, 522 paired specimens, including 388 from persons who were not tested previously, were tested.

The percentage positive agreement (PPA) and the percentage negative agreement (PNA) were determined for each sample, and the antigen test had an 84% to 99% PPA and a near 100% PNA. When antigen was compared to RT-PCR, 133 of 532 paired samples were positive by one of the methods. Of those who were positive, 64% were positive by both methods. Only 33 of 113 were RT-PCR positive and antigen negative. Of all the 532 paired samples, the PPA between antigen and PCR was 69% and the PNA was 98%. For those who were not positive previously, the PPA was 63%. Importantly, for all groups, the PNA between antigen and PCR remained near 100%.

Virus culture was not particularly sensitive in detection of virus in antigen-positive subjects. Viral cultures were positive when tested for only 21% of positive specimens. Viral culture was attempted only for subjects who were considered likely to have positive cultures (i.e., those with a cycle threshold [CT] of < 34). Antigen performed best with early infections compared to late infections, with PPA 86% in early infection and 51% in late infection. Antigen positivity generally related to lower CT values.

COMMENTARY

This study in a Georgia nursing home population among residents who had been infected and those who had not showed rapid antigen testing was useful in detecting infected persons. It was not quite as good as RT-PCR, but good enough to be used in the setting of such healthcare facilities. The critical issue is whether rapid antigen testing can be used to identify those residents or patients within health facilities who need to be cohorted or receive early treatment with monoclonal antibodies to reduce disease progression and virus spread. The Abbott BinaxNOW antigen kit was used in this study, and it will be interesting to see if newer antigen methodology can improve on the data reported in this article.

Because antigen testing was more sensitive, perhaps fortuitously, in early vs. late infection, future studies can focus on just how early rapid antigen testing could be used to reduce subsequent spread. The proliferation of virus likely peaks between two to four days, suggesting to the hospital epidemiologist that tools like point prevalence surveys at a given frequency, say weekly, may detect additional infected persons in facilities like this Georgia nursing home. False-positives were stated to number only eight at a time when PNA was 98%. Several of those false-positives were in residents who had tested positive by PCR. Rapid COVID-19 antigen detection likely will assume more use because of ease of use, low cost, and reliability that a negative result correlates well with more expensive time-consuming tests.