What are Rapid Diagnostic Tests?
Rapid Diagnostic Tests, or RDTs, are authorized by the Food and Drug Administration (FDA) and approved for use in persons with COVID-19 symptoms or with asymptomatic individuals who have been exposed to COVID-19. RDTs are either nucleic acid amplification tests (NAATs) which detect genes, or antigen-based immunoassays to detect proteins of SARS-CoV-2. Molecular NAATs detect the presence of viral gene targets, including the N, S, and E genes. Reverse-transcription-polymerase-chain-reaction assays, or RT-PCR tests are the most common type of COVID-19 NAATs and typically are performed in the lab and can take 1-3 days for results. The antigen-based tests detect domains of surface proteins specific to SARS-CoV-2; these can be bought in a local pharmacy, taken at home, and show results in 15 minutes. At home COVID-19 testing kits are examples of antigen-based tests. In general, NAATs or PCR tests are more sensitive than antigen-based tests because of their ability to amplify target genome sequences.
While NAATs are more accurate, they can show a positive test result weeks to months after infection. This is because NAATs can detect remnant viral RNA well after the time period of replication-competent virus; studies have found that COVID-19 generally replicates for only 10 to 14 days after symptom onset. Meanwhile, antigen-based tests remain positive for 5-12 days after symptom onset and are more accurate in those with a higher load. In other words, antigen-based tests are more accurate with replication component SARS-CoV-2 than molecular tests and thus may provide information about potential transmissibility. For a list of FDA approved RDTs, see here.
Interpreting Testing Results
For individuals exposed to COVID-19, testing is generally not useful within the first 48 hours of exposure since the virus will not have reached a sufficient viral load. Instead, testing should be done 5 to 7 days after exposure, which is the average peak of symptoms and viral load. Following FDA approved indication, a second RDT test should be performed 2 days after an initial negative test (which should be performed 5-7 days after exposure). It's also important to note that each type of test is affected differently by the different COVID-19 variants. For specific information on the impact of mutations on COVID-19 tests, see here.
The accuracy of a COVID-19 RDT test depends on the result itself and pretest probability of infection.For individuals with a moderate-to-high pretest probability, including symptomatic individuals or asymptomatic individuals with close contact to someone with COVID-19, a positive RDT indicates a confirmed COVID-19 infection. However, there is the possibility of false negatives and a second negative RDT result 2 days after the initial test or a negative laboratory based NAAT/PCR test helps rule out infection.
For those with a low pretest probability, for example asymptomatic individuals without exposure to COVID-19, a single negative RDT indicates that infection is unlikely. It's also possible for an RDT to indicate a false positive result; if there is low suspicion of infection, a second test should be performed. A second positive antigen-based or NAAT test confirms COVID-19 infection.
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Positive Test Results
Anyone who has a positive RDT should report their infection to a public health department. For asymptomatic persons who test positive, the CDC recommends quarantining for at least 5 days after a positive test. The use of a well-fitted mask is recommended for 5 days after the end of the isolation period. For symptomatic persons, a 10-day isolation period is recommended although a negative test is recommended for everyone before returning to public.
Conclusion
While antigen-based tests and nucleic acid amplification tests are both FDA approved rapid diagnostic tests, they differ in accuracy depending on how many days after exposure the test is taken. In general, it's best to take a second PCR test 2 days after the first COVID-19 test to confirm a testing result. Positive or not, it's important to continue to follow CDC guidelines and get vaccinated, especially with the fast spreading Omicron variant.
Sources
1. Drain, Paul K. “Rapid Diagnostic Testing for SARS-COV-2.” New England Journal of Medicine, vol. 386, no. 3, 20 Jan. 2022, pp. 264–272., https://doi.org/10.1056/nejmcp2117115.
2. Center for Devices and Radiological Health. “SARS-COV-2 Viral Mutations: Impact on COVID-19 Tests.” U.S. Food and Drug Administration, FDA, https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/sars-cov-2-viral-mutations-impact-covid-19-tests.
3. “Fact Sheet: The Biden Administration to Begin Distributing at-Home, Rapid Covid-19 Tests to Americans for Free.” The White House, The United States Government, 14 Jan. 2022, https://www.whitehouse.gov/briefing-room/statements-releases/2022/01/14/fact-sheet-the-biden-administration-to-begin-distributing-at-home-rapid-covid-19-tests-to-americans-for-free/.
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