The differences and the benefits
The virus spread around the world in a few short months, and its variants continue to sweep the globe. The good news is that a robust testing program as part of an organization’s SARS-CoV-2 response can help mitigate the spread of the disease. Diagnostic testing can identify active coronavirus infection and inform proactive next steps, like quarantining, to protect the infected individual and close contacts.
As testing is critical to diagnose and contain the spread of the virus, there are multiple methodologies available to yield a coronavirus testing result. The two types of diagnostic tests discussed here are molecular lab-based PCR, which detects viral RNA, and antigen-based testing, which detects viral proteins (the antigen). PCR and antigen testing are described more fully below to help make strategic decisions regarding community testing, planning, and implementation.
SARS-CoV-2 PCR and antigen tests: get the facts
Technology
There are different types of technologies used to test for SARS-CoV-2:
PCR test
- Detect viral RNA
- SARS-CoV-2 RNA is extracted from the sample (throat swab, nasal swab, or saliva sample)
- PCR—a type of nucleic acid amplification test—is then used to detect the viral genome
Antigen test
- Detect the antigen—in this case, viral proteins
- The saliva sample or nasal swab sample is added to a surface coated with antibodies that bind to specific viral proteins; this is used to create a signal that detects the virus
When to use
PCR test
- When you need accurate results with high confidence
- Ideal for diagnostic and population surveillance testing, especially in a high-volume setting
If you have symptoms and a negative antigen test result, get a PCR test for confirmation.1 If you have symptoms or are asymptomatic and have a positive antigen test result, get a PCR test for confirmation.
Antigen test
When you need convenience and speed to quickly determine if a person may have the virus
Ideal for point-of-care testing and screening high-risk congregate settings—not for asymptomatic populations 2,3
Accuracy
Accuracy takes into account sensitivity and specificity by measuring how the test can correctly identify if a sample is infected with SARS-CoV-2.
PCR test
- PCR tests have are considered the “gold standard” for detecting whether the virus is present
- Highly sensitive PCR tests can detect low viral loads, even in the absence of symptoms
Antigen test
- In one study, an antigen test had a sensitivity of 64% in symptomatic cases, meaning 36% (~2 in 5) positive cases receive a negative result (known as false negatives)*
- In the same study, the antigen test had a sensitivity of 36% in asymptomatic cases, meaning 64% (~3 in 5) of positive cases receive a negative result (false negatives)*
* cdc.gov/mmwr/volumes/70/wr/mm7003e3.html
Turnaround time
PCR test
- It can take as little as 24 hours to get results
Antigen test
- It can take up to 30 minutes to report out each individual result, especially if the result is negative
For the latest guidelines, please refer to:
The CDC:
https://www.cdc.gov/coronavirus/2019-ncov/lab/testing.html
https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html
The European CDC:
https://www.ecdc.europa.eu/en/publications-data/options-use-rapid-antigen-tests-covid-19-eueea-and-uk
The World Health Organization:
https://www.who.int/publications/i/item/antigen-detection-in-the-diagnosis-of-sars-cov-2infection-using-rapid-immunoassays
Testing technology and community needs
The decision to include PCR and antigen technologies in a testing program depends on the unique needs of the community. Importantly, community leaders should consider the organization’s tolerance of a wrong answer and the acceptability of that risk: what are the potential downstream effects of allowing an infected individual into the community because of a false negative result? Does convenience outweigh accuracy?
A clear understanding of the testing options is the first step in designing a testing program that will help keep the community safe. PCR tests, the gold standard for SARS-CoV-2 testing, provide high accuracy and therefore high confidence in the test results. Antigen tests are convenient when only a few results are needed, which may be advantageous in the point-of-care setting.
Resources
- Reading list: Recommended articles about COVID-19 testing technologies and asymptomatic testing
- Blog: Why qPCR is the gold standard for COVID-19 testing
- Infographic: COVID-19 PCR and Antigen Tests Infographic Request Form
- Guide: Helping keep your people safe: A guide to asymptomatic population testing options for SARS-CoV-2
- Infographic: SARS-CoV-2 Biology and COVID-19 Testing Infographic: Request Form
- Webinars: COVID-19 Testing Solutions Webinars
References
1. https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html
2. https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/covid-19-test-uses-faqs-testing-sars-cov-2
3. https://www.cms.gov/files/document/clia-sars-cov-2-point-care-test-enforcement-discretion.pdf