As the COVID-19 pandemic has surged into its second year, rapid tests to detect SARS-CoV-2 and deliver results quickly have become more prevalent.
The gold standard to diagnosis COVID-19 is polymerase chain reaction (PCR), but turnaround times for these tests have lagged at times during the pandemic for a variety of reasons, such as the sheer volume of specimens or shortages of supplies, testing platforms, and personnel.
As a result of inconsistent turnaround times for PCR tests, rapid tests have gained prominence during the pandemic. The number of rapid tests – which the Infectious Disease Society of America (IDSA) defines a rapid test as one that returns results in less than one hour – 1 have been increasing, too.
There are two major categories of rapid tests: molecular and antigen. While rapid antigen tests usually are antibody-based and capture SARS-CoV-2 antigens (typically N protein), the rapid molecular tests detect viral RNA.
The tests often can be performed at the point-of-care, such as a doctor’s office or pharmacy, reducing the number of steps between specimen collection and results. The tests also are moving into the home. For example, the U.S. Food and Drug Administration granted emergency use authorization (EUA) in November 2020 to the first COVID-19 test that consumers can perform completely at home.2
The Department of Health and Human Services (HHS) has said that rapid tests should supplement laboratory testing in certain situations, such as emerging outbreaks, or locations where laboratory-based tests are not readily available, such as rural areas. Using rapid testing also makes sense in situations, such as congregate living facilities, where people are tested often. This would include nursing homes, prisons, and college dormitories.
Indeed, the federal government has purchased and distributed rapid tests to screen people living in nursing homes, and it has developed rules about how often staff and residents should be tested.
In the case of the antigen tests specifically, their speed, simplicity and low cost could foster better containment of the virus by allowing millions of people to be tested daily, increasing the number of infected people who quarantine and reducing of spread of the virus.
However, antigen tests tend to be less sensitive than molecular tests. And some studies have suggested that the antigen tests may not detect the virus in patients who have had symptoms for more than five days, leading to concerns about how well the tests perform, particularly among asymptomatic people.1
According to the Centers for Disease Control and Prevention (CDC), the specificity of antigen tests is generally as high as most molecular tests, which means that false positive test results are not likely if the manufacturer’s instructions are followed correctly. The CDC also notes, however, that false positive results may occur in communities where the rate of infection is low, which is true for all in vitro tests.3 The agency defines low prevalence as when the positivity rate for NAAT tests is less than 5 percent, or where there are fewer than 20 new cases per 100,000 people within the last 14 days.3 (See chart below).
References
- Rapid testing. COVID-19 real time learning network. Infectious Disease Society of America. December 2020. https://www.idsociety.org/covid-19-real-time-learning-network/diagnostics/rapid-testing/. Accessed February 8, 2021.
- FDA grants EUA for at-home test. Medical Laboratory Observer. November 2020. https://www.mlo-online.com/diagnostics/assays/article/21163382/fda-grants-eua-for-athome-test. Accessed February 8, 2021.
- Interim guidance for antigen testing for SARS-CoV-2. Centers for Disease Control and Prevention. Updated December 2020. https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html. Accessed February 8, 2021.
