Overview of Testing for SARS-CoV-2, the virus that causes COVID-19

Key Points

• People who have symptoms of COVID-19 or who have had known exposure to someone with COVID-19 should be tested for SARS-CoV-2 infection.
• Screening testing may be appropriate in some specific settings.

This overview describes current information on the types of tests used to detect SARS-CoV-2 infection and their intended uses. This information is intended for use by healthcare providers, public health professionals, and those organizing and implementing testing in non-healthcare settings. Information for the general public on COVID-19 testing is also available.

Considerations When Testing

People who are tested for COVID-19 should receive clear information on:

• The purpose of the test
• Whether the test is a nucleic acid amplification test (NAAT) such as a PCR test, or an antigen test
• Who will pay for the test
• How the test will be performed
• How and when they will receive test results
• How to understand what the results mean
• What actions need to happen after someone has a negative or positive result
• The performance specifications and any limitations associated with the test
• The difference between diagnostic testing and screening testing
• Who will receive the results and how they may be used
• Any consequences for declining to be tested
• The manufacturer, test name, and type of the test

Individuals tested are required to receive patient fact sheets as part of the test’s Emergency Use Authorization (EUA).

Testing for SARS-CoV-2 Infection

Test Types

Viral tests, including Nucleic Acid Amplification Tests (NAATs, such as PCR tests), antigen tests and other tests (such as breath tests) are used as diagnostic tests to detect current infection with SARS-CoV-2, determine the need for prevention measures like isolation, and to inform an individual’s medical care. Viral tests can also be used as screening tests.

Viral Tests include:

• Nucleic Acid Amplification Tests (NAATs) are highly sensitive and highly specific tests that detect one or more viral ribonucleic acid (RNA) genes. PCR tests are the most common type of NAAT used for COVID-19 testing. Viral RNA may stay in a person’s body for up to 90 days after they test positive. Therefore, NAATs should not be used to test someone who has tested positive in the last 90 days. Most NAATs need to be performed in a laboratory, although some are performed at the point-of-care. Most NAATs produce qualitative (positive/negative) results.
• Antigen tests are immunoassays that detect the presence of specific viral proteins, called antigens. A positive test indicates current infection. Antigen tests generally have high specificity, similar to NAATs, but are less sensitive than most NAATs. Because antigen tests have lower sensitivity, FDA recommends negative antigen tests be repeated up to three times, with each test 48 hours apart to confirm a negative result. Most antigen tests are less expensive than NAATs and can provide results in minutes. Antigen tests are available for at-home testing (self-testing), at the point of care, or in a laboratory.
  • As noted in the labeling for authorized over-the-counter antigen tests: Negative results should be treated as presumptive (meaning that they are preliminary results). Negative results do not rule out SARS-CoV-2 infection and should not be used as the sole basis for treatment or patient management decisions, including infection control decisions. Please see FDA guidance on the use of at-home COVID-19 antigen tests.
• Other diagnostic tests may be used to detect SARS-CoV-2 from non-traditional respiratory specimens, such as breath. These tests’ results may be presumptive and require confirmation by NAAT. Please refer to each test’s Instructions for Use (IFU) for specific interpretation.

Positive viral test results indicate current, or sometimes recent, infection and the person with COVID-19 should follow CDC recommendations for isolation.

Negative viral test results mean the test did not detect the virus, but this doesn’t rule out that you could have an infection. These results represent a snapshot of the time around specimen collection and could change if the same test was performed again in one or more days. Negative antigen test results should be repeated following FDA guidance.

Antibody (or serology) tests are used to test for the presence of antibodies from previous infection or vaccination and can aid in fulfilling the case definition for multisystem inflammatory syndrome in children (MIS-C) and adults (MIS-A)². Antibody testing does not diagnose current infection. Antibody testing is primarily used for public health surveillance and epidemiologic purposes. Antibody tests detect specific antibodies that target different parts (nucleocapsid or spike protein) of the virus. Detection of anti-nucleocapsid antibody indicates SARS-CoV-2 infection, while anti-spike protein antibody may be induced by COVID-19 vaccination or by SARS-CoV-2 infection. This should be considered when choosing whether to test for antibodies originating from past infection versus those from vaccination.

See FDA’s list of In Vitro Diagnostics Emergency Use Authorizations for more information about the performance and interpretation of specific authorized tests.

Overview of Testing Scenarios

Diagnostic testing is intended to identify current infection. It is performed when a person has signs or symptoms consistent with COVID-19 or is asymptomatic but has recent known or suspected exposure to someone with COVID-19.

Screening testing is intended to identify people with COVID-19 who are asymptomatic or do not have any known, suspected, or reported exposure to someone with COVID-19.

Public health surveillance testing is conducted to specifically monitor population-level burden of disease, or to characterize the incidence and prevalence of SARS-CoV-2 infection. Surveillance testing is primarily used to gain information at a population level, rather than an individual level, and involves testing of de-identified specimens. Surveillance testing results are not reported back to the individual. As such, surveillance testing cannot be used for an individual’s healthcare decision-making or individual public health actions, such as isolation. Examples of public health surveillance testing are genomic surveillance and wastewater surveillance.

Choosing a Test

When choosing which test to use, it is important to understand the purpose of the testing (diagnostic or screening), test performance in context of current COVID-19 incidence, need for rapid results, and other considerations (See Table 1).  Positive predictive values (probability that the person testing positive is actually infected) and negative predictive values (probability that the person testing negative is actually not infected) of NAAT and antigen tests vary depending upon the pretest probability. Pretest probability considers both the prevalence of COVID-19 in the community as well as the clinical context of the individual being tested. CDC provides general information on sensitivity, specificity, positive and negative predictive values for antigen tests and antibody tests. For information on a specific test, refer to FDA’s website.

Table 1 summarizes some characteristics of NAATs and antigen tests to consider for a testing program. Laboratories that perform screening or diagnostic testing for SARS-CoV-2 must have a CLIA certificate and meet regulatory requirements. Tests that have received an EUA from FDA for point-of-care (POC) use can be performed with a CLIA certificate of waiver.

Vaccination and SARS-CoV-2 Testing

Vaccination does not affect the results of someone’s SARS-CoV-2 diagnostic or screening tests (nucleic acid amplification tests [NAAT], antigen or other diagnostic tests).

The main effect of vaccination on SARS-CoV-2 testing is related to antibody testing. Because mRNA COVID-19 vaccines use the SARS-CoV-2 spike protein to generate an immune response, a positive serologic (antibody) test for spike protein IgM/IgG could indicate either previous infection or vaccination.

Antibody testing is not currently recommended to assess a person’s protection against SARS-CoV-2 infection or severe COVID-19 following COVID-19 vaccination or prior infection, or to assess the need for vaccination in an unvaccinated person. Antibody testing can be used in the diagnosis of Multisystem Inflammatory Syndrome in Children (MIS-C) or Multisystem Inflammatory Syndrome in Adults (MIS-A).

To evaluate for evidence of previous infection in a vaccinated individual, use an antibody test specifically evaluating IgM/IgG to the nucleocapsid protein. For example, specific antibody tests can be used for public health surveillance.

Health Equity in SARS-CoV-2 Testing

Social determinants of health may influence access to testing.  For example, travel time may limit access to, and use of, testing services for those who have limited access to transportation and who live in areas with fewer public transit services and schedules. Racial and ethnic disparities in test site distribution have been found.³ Other factors that may affect both access to, and use of, testing services include:

• lack of health insurance
• concern about the costs or co-pays
• occupational factors such as not being able to take time off work and lack of paid leave
• lack of accessible options for people with disabilities, and
• distrust of the government and healthcare systems.^{4, 5, 6, 7}

Delays in testing may also delay seeking care and treatment (when sick) as well as delays in self-isolation that could reduce the spread of the virus to others.

CDC’s COVID-19 Response Health Equity Strategy outlines a plan to reduce the disproportionate burden of COVID-19 among people in some racial and ethnic minority groups, people with disabilities, and other population groups (e.g., essential and frontline workers, people living in rural or frontier areas) who have experienced a disproportionate burden of COVID-19. One component to move towards greater health equity is ensuring availability of resources, including access to testing for populations who have experienced longstanding, systemic health and social inequities. All population groups, including racial and ethnic minority groups, should have equal access to affordable, quality, and timely SARS-CoV-2 testing—with fast turnaround time for results—for diagnosis and screening. Efforts should be made to address barriers that might overtly or inadvertently create inequalities in testing.

In addition, completeness of race and ethnicity data is an important factor in understanding the impact the virus has on racial and ethnic minority populations. When possible, healthcare providers and public health professionals should ask and record race and ethnicity for anyone receiving a reportable test result and ensure these data are reported with the person’s test results to facilitate understanding the impact of COVID-19 on racial and ethnic minority populations.

Some strategies to achieve health equity in testing access and availability include:

• Use a social vulnerability index to assist in selecting testing sites.
• Increase the availability of free testing sites in communities. Employers, community-based, and faith-based organizations can be important partners to increase the number of free, community-based testing sites. This expansion ensures that wait times both for testing and reporting of results are decreased.
• Increase accessible and culturally appropriate public messaging about the importance of testing and communicate these messages in multiple accessible formats, languages, and venues, particularly in communities at higher risk and disproportionately impacted by the virus.

Considerations for testing in different scenarios

Diagnostic testing

Testing individuals with signs or symptoms consistent with COVID-19

Positive test results using a viral test (NAAT, antigen or other tests) in individuals with signs or symptoms consistent with COVID-19 indicate that the person has COVID-19. A negative antigen test in individuals with signs or symptoms of COVID-19 should be repeated following FDA recommendations or confirmed by NAAT. For more information, see Antigen Test Algorithm.

Additionally, consider other illnesses with similar symptoms that may require testing. For many diseases, including flu, early diagnosis and prompt treatment is very important for preventing severe illness.

Anyone who tests positive should isolate at home or, if in a healthcare setting, be placed on appropriate precautions. Some people should receive treatment. Most people with COVID-19 have mild illness and can recover at home. For more information, see CDC’s COVID-19 isolation guidance.

Testing asymptomatic people who have had recent known or suspected exposure to SARS-CoV-2

Viral Testing is recommended for individuals who have been exposed to someone with COVID-19. People who have had an exposure to someone known or suspected of having COVID-19 should be tested at least 5 days after the exposure. If symptoms develop before 5 days, they should get tested immediately.

People with a positive test result should follow CDC’s COVID-19 isolation guidance.

Testing people who have recently tested positive, and recovered from COVID-19

If someone has had exposure to someone with COVID-19 and is asymptomatic, but has had COVID-19 within the past 30 days*, testing to identify a new infection is generally not recommended. If someone has become newly symptomatic after having had COVID-19 within the past 30 days*, antigen tests should be used to identify a new infection. If they test negative, they should repeat the antigen test following FDA recommendations.

If someone had exposure to another person with COVID-19, but the exposed individual has had COVID-19 within the past 31-90 days*, consider using antigen tests (rather than an NAAT, such as a PCR test) to identify a new infection.  They should get tested at least 5 full days after their exposure.  If they test negative with an antigen test, they should repeat the antigen test following FDA recommendations.

*The clock starts from the day the person is tested (not the day they received their positive test result) or their original onset of symptoms, whichever came first.

Some adults with severe illness or who are moderately or severely immunocompromised may produce replication-competent virus beyond 10 days that may warrant extending duration of isolation and precautions. A test-based strategy for ending isolation in these patients may be considered in consultation with infectious disease experts. For more information, including on retesting people previously infected with SARS-CoV-2, visit Ending Isolation and Precautions for People with COVID-19: Interim Guidance.

Screening Testing

Testing asymptomatic people without recent known or suspected exposure to SARS-CoV-2 for early identification, isolation, and disease prevention

Screening testing allows early identification and isolation of people who are asymptomatic or pre-symptomatic and who might be unknowingly transmitting virus. Screening testing may be most valuable in specific settings where early identification is essential to reducing transmission and mitigating risk for severe disease among populations at high risk. When COVID-19 hospital admission levels are high, CDC recommends implementing screening testing in certain high-risk settings.

Examples of screening testing include:

• Point-in-time screening testing
  • This is screening testing that happens on a situational basis to screen a person, or group of people, for COVID-19 at a single time point.
• Serial screening testing
  • This is screening testing that is repeated at different points in time within a group, such as testing every 3 days for everyone in a particular setting or facility.

How to conduct screening testing

When screening testing is used, it should be applied to participants regardless of vaccination status.

Any type of viral test can be used for screening purposes; however, consider the characteristics (including accessibility, accuracy and efficiency) of different test types to determine which best suits screening testing needs.

People without symptoms and without known exposure to COVID-19 do not need to take any special actions while awaiting screening test results.

Settings to prioritize for screening testing

Settings that should be prioritized for screening testing include facilities and situations where transmission risk is high and the population served is at high risk of severe outcomes from COVID-19 or there is limited access to healthcare, including:

• Specific settings that have demonstrated high potential for rapid and widespread virus spread to people at higher risk for severe illness.
• Settings that involve close quarters and that are isolated from healthcare resources (e.g., fishing vessels, wildland firefighter camps, or offshore oil platforms).

Serial screening testing is less effective at reducing COVID-19’s impacts in settings where disease rates are lower, risk of spread is lower, and risk of severe illness is lower. Because of this, CDC does not recommend serial screening testing in most lower risk settings.

Public Health Surveillance Testing for SARS-CoV-2

Public health surveillance testing may sample a certain percentage of a specific population to monitor for increasing or decreasing infection rate or to determine the population effect from community interventions. An example of public health surveillance testing is when a state public health department samples a random percentage of all people in a city on a rolling basis to assess local infection rates and trends.

CDC is working with state, local, territorial, academic, and commercial partners to conduct surveillance testing to better understand COVID-19 in the United States.

For more on surveillance conducted by CDC:

• Data and Surveillance
  • COVID-19-Associated Hospitalization Surveillance Network (COVID-NET)
  • National Wastewater Surveillance System (NWSS)
  • FAQ: COVID-19 Data & Surveillance
• Surveillance and Data Analytics
• CDC’s Role in Tracking Variants

Additional Information

• Setting-specific Testing Guidance
  Infection Prevention and Control Recommendations for Healthcare Personnel
  K-12 Schools
  Homeless Shelters, Correctional and Detention Facilities
• Testing Information for the Public
  COVID-19 Testing: What You Need to Know
  Self-Testing
• Other Testing Resources
  Antigen Testing Algorithm
  Interim Guidelines for COVID-19 Antibody Testing
  Pooled Procedures for Testing
  Laboratory Resources

Previous Updates

References

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3. Dalva-Baird NP, Alobuia WM, Bendavid E, Bhattacharya J. Racial and ethnic inequities in the early distribution of U.S. COVID-19 testing sites and mortality. Eur J Clin Invest. 2021;51(11):e13669. doi:10.1111/eci.13669
4. Economic Policy Institute. Black Workers Face Two of the Most Lethal Preexisting Conditions for Coronavirus—Racism and Economic Inequality [online]. 2020 [cited 2020 Jun 28]. Available from URL: https://www.epi.org/publication/black-workers-covid/
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7. Institute of Medicine (US) Committee on Understanding and Eliminating Racial and Ethnic Disparities in Health Care, Smedley BD, Stith AY, Nelson AR, eds. Unequal Treatment: Confronting Racial and Ethnic Disparities in Health Care. Washington (DC): National Academies Press (US); 2003.
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