NIH study suggests rapid antigen and PCR tests for COVID-19 are equally reliable when used frequently

Amid the continued deluge of emergency use authorizations from the FDA, rapid antigen tests for COVID-19 are more widely available and easily accessible than ever.



On a one-off basis, these diagnostics—which can return results in as few as 15 minutes—have a lower sensitivity rate compared to molecular-based PCR tests, which must be processed in a lab for analysis. The gap between them narrows, however, when either test is used as part of a regular screening regimen, according to a new study from the National Institutes of Health (NIH).



Researchers from the NIH’s Rapid Acceleration of Diagnostics initiative, or RADx, compared antigen and PCR test results of 43 people with COVID who were given both tests every day for two weeks.



Each day, the participants provided saliva samples for analysis with the covidSHIELD PCR test developed by the University of Illinois at Urbana-Champaign, as well as nasal swabs to be tested with both Abbott’s Alinity PCR assay and Quidel’s Sofia antigen test.



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In calculations of the overall sensitivity of each type of test, the researchers found that both the PCR and antigen analyses achieved more than 98% sensitivity when results from every three days were taken into account.



That’s a stark improvement from the 80% sensitivity rating calculated for once-weekly antigen tests. (The PCR tests still resulted in about 98% sensitivity even when administered only once per week.)



“Rapid antigen testing at home, two to three times per week, is a powerful and convenient way for individuals to screen for COVID-19 infection,” said Bruce Tromberg, head of the RADx program and director of the NIH's National Institute of Biomedical Imaging and Bioengineering.



“With schools and businesses reopening, an individual’s risk of infection can change from day to day. Serial antigen testing can help people manage this risk and quickly take action to prevent spread of the virus,” Tromberg continued.



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PCR tests, commonly referred to as the “gold-standard” COVID diagnostic, use a thermal cycler machine in a laboratory to extract any genetic material from the coronavirus that may be present in a saliva or nasal sample.



Because of the high-cost professional equipment required to run these analyses—and the fact that they typically require at least a one-day turnaround—PCR tests have not been able to be used for widespread, routine screening initiatives, despite their high accuracy levels.



Antigen tests, meanwhile, look for specific protein markers attached to the outside of the virus and can return results in minutes using small, portable devices. The accuracy of those results can vary widely, however, with rapid antigen tests far more likely than PCR assays to return false negatives.



Alongside the PCR and antigen tests, the researchers also sent samples to a Johns Hopkins University lab to determine when the virus was actually live and transmissible.



When the daily test results were compared to these data, the researchers were able to conclude that both PCR and antigen tests are most accurate when the live virus is present. And while a single PCR test did a better job than a standalone antigen analysis at detecting COVID both before and after the infectious period, antigen testing was still able to match those sensitivity levels when conducted two or three times per week.




The findings may come as good news to Abbott, which had been banking on portable antigen test sales to provide a windfall this year, as the U.S. continues to transition toward long-term coronavirus monitoring. However, the success of COVID shots, and recent guidelines from the Centers for Disease Control and Prevention saying fully vaccinated people can skip some tests, have led to those markets drying up faster than expected—leading Abbott to cut back its 2021 financial forecasts by as much as $2 billion.