Covid-19

Overview of Testing for SARS-CoV-2 (COVID-19)



https://www.cdc.gov/coronavirus/2019-ncov/images/social-media/covid-hcp-sm.png

Considerations When Testing

SARS-CoV-2 testing may be incorporated as part of a comprehensive approach to reducing transmission. Symptom screening, testing, and contact tracing are strategies to identify people infected with SARS-CoV-2 so that actions can be taken to slow and stop the spread of the virus.

People undergoing testing should receive clear information on

  • The manufacturer and name of the test, the type of test, the purpose of the test, the performance specifications of the test, any limitations associated with the test, who will pay for the test, how the test will be performed, how and when people will receive test results, and;
  • How to understand what the results mean, actions associated with negative or positive results, the difference between testing for workplace screening versus for medical diagnosis, who will receive the results, how the results may be used, and any consequences for declining to be tested.

Individuals tested are required to receive patient fact sheets as part of the test’s emergency use authorization (EUA)external icon.

Vaccination and SARS-CoV-2 Testing

Prior receipt of a COVID-19 vaccine should not affect the results of SARS-CoV-2 viral tests (nucleic acid amplification tests (NAAT) or antigen). Because the Pfizer-BioNTech, Moderna, and Johnson & Johnson 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 for immunity to SARS-CoV-2 infection following COVID-19 vaccination or to assess the need for vaccination in an unvaccinated person. To evaluate for evidence of previous infection in a vaccinated individual, an antibody test specifically evaluating IgM/IgG to the nucleocapsid protein should be used (e.g., for public health surveillance or the diagnosis of Multisystem Inflammatory Syndrome in Children (MIS-C) Multisystem Inflammatory Syndrome in Adults (MIS-A)). For guidance on quarantine and testing of fully vaccinated people, please visit Interim Public Health Recommendations for Fully Vaccinated People.

Testing for SARS-CoV-2 Infection

Many categories of tests are used to detect SARS-CoV-2,1 and their performance characteristics vary.

The selection and interpretation of SARS-CoV-2 tests should be based on the context in which they are being used, including the prevalence of SARS-CoV-2 in the population being tested (See Table 1) and the status (signs, symptoms, contacts) of the person being tested.

Test Types

Viral tests, including NAATs and antigen tests, are used as diagnostic tests to detect infection with SARS-CoV-2 and to inform an individual’s medical care. Viral tests can also be used as screening tests to reduce the transmission of SARS-CoV-2 by identifying infected persons who need to isolate from others. See FDA’s list of In Vitro Diagnostics Emergency Use Authorizationsexternal icon for more information about the performance of specific authorized tests.

  • NAATs are high-sensitivity, high-specificity tests for diagnosing SARS-CoV-2 infection. NAATs detect one or more viral ribonucleic acid (RNA) genes and indicate a current infection or a recent infection but, due to prolonged viral RNA detection, are not always direct evidence for the presence of virus capable of replicating or being transmitted to others. Most NAATs need to be processed in a laboratory, and time to results can vary (~1–3 days), but some NAATs are point-of-care tests with results available in about 15–45 minutes. Most NAATs produce qualitative results. NAATs can be performed on upper respiratory specimens, such as nasopharyngeal, nasal mid-turbinate, anterior nasal, or saliva.
  • Antigen tests are immunoassays that detect the presence of a specific viral antigen. Antigen tests generally have similar specificity, but are less sensitive than most NAATs. Most are less expensive than NAATs and can be processed at the point of care with results available in minutes and thus can be used in screening programs to quickly identify those who are likely to be contagious. Because of the performance characteristics of antigen tests, it may be necessary to confirm some antigen test results (a negative test in persons with symptoms or a positive test in persons without symptoms) with a laboratory-based NAAT. However, based on the authorization from FDAexternal icon, some point-of-care NAATsexternal icon that provide presumptive results cannot be used for confirmatory testing. Use of the Antigen Testing Algorithm is recommended to determine when confirmatory testing is needed. Antigen tests can be performed on nasopharyngeal or anterior nasal specimens.

Correct interpretation of results from both antigen tests and confirmatory NAATs, when indicated, is important.

Positive test results allow for identification and isolation of infected persons, as well as a case interview to identify and notify the case’s close contact(s) of exposure and the need to quarantine.

Negative test results in persons with known SARS-CoV-2 exposure suggest no current evidence of 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. Unvaccinated individuals with a negative result should continue to quarantine for 14 days or for the period established by local public health authorities. Fully vaccinated people should be tested 5–7 days after their last exposure. For guidance on quarantine and testing of fully vaccinated people, visit Interim Public Health Recommendations for Fully Vaccinated People for more information. In healthcare facilities with an outbreak of SARS-CoV-2, recommendations for viral testing of healthcare providers, residents, and patients (regardless of vaccination status) remain unchanged.

Negative test results in persons without symptoms and no known exposure suggest no infection. All persons being tested, regardless of results, should receive counseling on the continuation of risk reduction behaviors that help prevent the transmission of SARS-CoV-2 (e.g., wearing masks, physical distancing, avoiding crowds and poorly ventilated spaces).

Antibody (or serology) tests are used to detect previous infection with SARS-CoV-2 and can aid in the diagnosis of Multisystem Inflammatory Syndrome in Children (MIS-C) and in adults (MIS-A)2. CDC does not recommend using antibody testing to diagnose current infection. Depending on the time when someone was infected and the timing of the test, the test might not detect antibodies in someone with a current infection. In addition, it is not currently known whether a positive antibody test result indicates immunity against SARS-CoV-2; therefore, at this time, antibody tests should not be used to determine if an individual is immune against reinfection. Antibody testing is being used for public health surveillance and epidemiologic purposes. Because antibody tests can have different targets on the virus, specific tests might be needed to assess for antibodies originating from past infection versus those from vaccination. For more information about COVID-19 vaccines and antibody test results, refer to Interim Clinical Considerations for Use of mRNA COVID-19 Vaccines Currently Authorized in the United States.

Overview of Testing Scenarios

Diagnostic testing is intended to identify current infection in individuals and is performed when a person has signs or symptoms consistent with COVID-19, or is asymptomatic, but has recent known or suspected exposure to SARS-CoV-2.

Examples of diagnostic testing include:

  • Testing persons with symptoms consistent with COVID-19, whether or not they are vaccinated
  • Testing persons as a result of contact tracing efforts
  • Testing persons who indicate that they were exposed to someone with a confirmed or suspected case of COVID-19.

Screening tests are recommended for unvaccinated people to identify those who are asymptomatic and do not have known, suspected, or reported exposure to SARS-CoV-2. Screening helps to identify unknown cases so that measures can be taken to prevent further transmission.

Examples of screening include:

  • Testing employees in a workplace setting
  • Testing students, faculty, and staff in a school or university setting
  • Testing a person before or after travel
  • Testing at home for someone who does not have symptoms associated with COVID-19 and no known exposures to someone with COVID-19

Public health surveillance is intended to monitor population-level burden of disease, or to characterize the incidence and prevalence of disease. Surveillance testing is primarily used to gain information at a population level, rather than an individual level, and generally 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 or quarantine.

An example of surveillance testing is wastewater surveillance.

Choosing a Test

When choosing which test to use, it is important to understand the purpose of the testing (diagnostic or screening), performance of the test within the context of the level of community transmission, need for rapid results, and other considerations (See Table 1). For example, even a highly specific antigen test may have a poor positive predictive value (high number of false positives) when used in a community where prevalence of infection is low. As an additional example, use of a laboratory-based NAAT in a community with high transmission and increased test demand may result in diagnostic delays due to processing time and time to return results. Positive and negative predictive values of NAAT and antigen tests vary depending upon the pretest probability. Pretest probability considers both the prevalence of the level of community transmission as well as the clinical context of the individual being tested. Additional information on sensitivity, specificity, positive and negative predictive values for antigen tests and antibody tests, and for the relationship between pretest probability and the likelihood of positive and negative predictive values pdf icon is available. Also see FDA’s letters to clinical laboratory staff and healthcare providers on the potential for false-positive results with antigen tests and the potential for false-negative results with molecular tests if a genetic variant of SARS-CoV-2 occurs in the part of the viral genome assessed by the test.

Table 1 summarizes some characteristics of NAATs and antigen tests to consider for a testing program. Given the risk of transmission of SARS-CoV-2 from asymptomatic and presymptomatic persons with SARS-CoV-2 infection, use of antigen tests in asymptomatic and presymptomatic persons can be considered. FDA has provided a list of FAQs for healthcare providers who are using diagnostic tests in screening asymptomatic individuals, and the Centers for Medicare & Medicaid Services will temporarily exercise enforcement discretion to enable the use of antigen tests in asymptomatic individuals for the duration of the COVID-19 public health emergency under the Clinical Laboratory Improvement Amendments of 1988 (CLIA). 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.

vials light icon

COVID-19 Viral Testing Tool

A tool to help healthcare providers quickly access the most relevant, actionable information to determine what type(s) of COVID-19 testing they should recommend. After test results are in, the tool can help interpret test results and guide next steps.

Table 1. NAAT and Antigen Test Differences to Consider When Planning for Diagnostic or Screening Use

Viral Ribonucleic Acid (RNA)

Nasal, Nasopharyngeal, Oropharyngeal, Sputum, Saliva

Varies by test, but generally high for laboratory-based tests and moderate-to-high for POC tests

Varies depending on the course of infection, but generally moderate-to-high at times of peak viral load*

Authorized for Use at the Point-of-Care

Most 1-3 days. Some could be rapid in 15 minutes

Ranges from 15 minutes to 30 minutes

Moderate (~$75-$100/test)

Most sensitive test method available

Short turnaround time for NAAT POC tests, but few available

Usually does not need to be repeated to confirm results

Short turnaround time (approximately 15 minutes)+

When performed at or near POC, allows for rapid identification of infected people, thus preventing further virus transmission in the community, workplace, etc.

Comparable performance to NAATs in symptomatic persons and/or if culturable virus present, when the person is presumed to be infectious

Longer turnaround time for lab-based tests (1–3 days)

Higher cost per test

A positive NAAT diagnostic test should not be repeated within 90 days, because people may continue to have detectable RNA after risk of transmission has passed

May need confirmatory testing

Less sensitive (more false negative results) compared to NAATs, especially among asymptomatic people

*The decreased sensitivity of antigen tests might be offset if the POC antigen tests are repeated more frequently (i.e., serial testing at least weekly).
^ Costs for: NAATsexternal icon
+Refers to point-of-care antigen tests only.

Health Equity in SARS-CoV-2 Testing

CDC’s COVID-19 Response Health Equity Strategy outlines a plan to reduce the disproportionate burden of COVID-19 among racial and ethnic minority populations 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 and to stop transmission of SARS-CoV-2 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 to reduce community transmission. 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. The U.S. Department of Health and Human Services has required laboratories and testing facilities to reportexternal icon race and ethnicity data to health departments, in addition to other data elements, for individuals tested for SARS-CoV-2 or diagnosed with COVID-19. Healthcare providers and public health professionals need to 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 in order to facilitate understanding the impact of COVID-19 on racial and ethnic minority populations.

In communities with a higher proportion of racial and ethnic minority populations and other populations disproportionately affected by COVID-19, health departments should ensure there is timely and equitable access to and availability of testing with fast result return, especially when the level of community transmission is substantial or high.

Some strategies to achieve this goal include:

  • Use a social vulnerability index to assist in selecting testing sites.
  • Assess the capacity of these sites to expand diagnostic and screening testing to meet the demand for impacted areas. This includes assessing the availability of free testing, wait times for testing and for results, and categories of available test (NAAT vs. antigen), as well as identifying and removing barriers to testing (e.g., alternatives to drive-through testing for a community where many do not have cars; availability of testing on evenings and weekends).
  • 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, helping limit the spread of SARS-CoV-2.
  • Increase public messaging about the importance of testing and communicate these messages in multiple languages and venues, particularly in communities at higher risk and disproportionately impacted by the virus.



Source link

Products You May Like

Articles You May Like

The Kids Aren’t Alright – The Health Care Blog
Occupational health disparities: The pandemic as prism and prod
U.S. International COVID-19 Vaccine Donations Tracker – Updated as of November 23
5 Ways To Make Your Company Office Safer
Symptoms of COVID-19 | CDC

Leave a Reply

Your email address will not be published. Required fields are marked *