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COVID-19 Rapid Tests Perform on Par with PCR Tests, Finds New Study

By LabMedica International staff writers
Posted on 02 Jul 2021
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Image: A vial of saliva sample for SARS-CoV-2 testing (Photo courtesy of Fred Zwicky, University of Illinois)
Image: A vial of saliva sample for SARS-CoV-2 testing (Photo courtesy of Fred Zwicky, University of Illinois)
Rapid antigen tests perform on par with lab tests when used every three days, according to a new screening study, thus building the case for frequent COVID-19 antigen testing.

Researchers at the National Institutes of Health (NIH; Bethesda, MD, USA) have found that detecting SARS-CoV-2, the virus that causes COVID-19, improves with regularity of testing, whether using rapid antigen tests or PCR molecular tests. The PCR test is considered the gold standard for diagnosing COVID-19 infection, but cost and infrastructure issues, as well as wait times for PCR results, have limited its use more broadly as a screening tool for asymptomatic people because rapid results are needed to interrupt the chain of transmission.

In the highly anticipated study, researchers affiliated with the NIH’s Rapid Acceleration of Diagnostics (RADx) initiative compared rapid antigen and laboratory PCR approaches for COVID-19 serial screening and reported results from 43 people infected with the virus. The researchers compared three COVID-19 viral testing modalities - PCR testing of saliva, PCR testing of nasal samples and rapid antigen testing of nasal samples. The researchers calculated the sensitivity of each test modality to detect SARS-CoV-2 and measured the presence of live virus over a two-week period following initial infection. They found that PCR molecular tests - both from saliva and nasal samples - are more sensitive than rapid antigen tests at detecting the SARS-CoV-2 virus prior to the infectious period. If the result from PCR tests could be quickly returned, the person receiving the result could undertake measures much sooner to prevent transmitting the virus to others. Unfortunately, results from PCR are rarely returned the day of testing.

The authors calculated test sensitivity based on test frequency, finding that a cadence of tests every three days achieved better than 98% sensitivity to detect infection, whether using rapid antigen tests or PCR tests. When they assessed frequency of testing once per week, nasal and saliva PCR testing sensitivity remained high, at around 98%, but antigen test sensitivity declined to 80%. These results show, for the first time, that testing at least twice per week with rapid antigen tests has comparable performance with PCR testing and maximizes the likelihood of detecting people infected with SARS-CoV-2. The sensitivity of PCR molecular tests and rapid antigen tests is highest when viral cultures are positive for SARS-CoV-2, as might be expected. Even beyond this infectivity period, though, PCR tests continue to detect particles of virus, when the virus is most likely no longer transmissible. Because antigen tests at the point of care or at home can deliver immediate results and are less costly than laboratory tests, these results suggest that they could be a highly effective screening tool to prevent disease outbreaks.

“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, Ph.D., director of the National Institute of Biomedical Imaging and Bioengineering (NIBIB), part of NIH. “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.”

“Silent transmission of the SARS-CoV-2 virus from individuals with no symptoms contributes significantly to the spread of the virus,” said co-author William Heetderks, M.D., Ph.D., a RADx Tech program advisor at NIBIB. “Faster, cheaper and broader testing with antigen tests can be a big help in the kind of large-scale screening scenarios that can find these silent transmitters.”

Related Links:
National Institutes of Health

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