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Testing for SARS-CoV-2 Infection

Last Updated: November 3, 2020

Testing for SARS-CoV-2 Infection
Summary Recommendations
  • The COVID-19 Treatment Guidelines Panel (the Panel) recommends that a nucleic acid amplification test (NAAT) for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) be used to diagnose acute infection (AIII).
  • The Panel recommends against the use of serologic testing as the sole basis for diagnosis of acute SARS-CoV-2 infection (AIII).
  • The Panel recommends against the use of serologic testing to determine whether a person is immune to SARS-CoV-2 infection (AIII).
Rating of Recommendations: A = Strong; B = Moderate; C = Optional
Rating of Evidence: I = One or more randomized trials with clinical outcomes and/or validated laboratory endpoints; II = One or more well-designed, nonrandomized trials or observational cohort studies; III = Expert opinion

Diagnostic Testing for SARS-CoV-2 Infection

Testing to diagnose severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection (i.e., using a nucleic acid amplification test [NAAT] or antigen test to detect SARS-CoV-2) should be done in all persons with symptoms that are consistent with COVID-19 and in people with known high-risk exposures to SARS-CoV-2. Ideally, diagnostic testing should also be performed in people who are likely to be at repeated risk of exposure, such as health care workers and first responders. Testing should be considered for individuals who spend time in environments with high population densities (e.g., those who work at or attend a school in person, those who work in the food industry) and for travelers who are returning from high-risk areas. Testing requirements may vary by state, local, and employer policies. For individuals who are planning to travel to other states or countries, documentation of a recent negative test result may be required for entry; in some cases, the documentation may be an acceptable alternative to undergoing a quarantine period upon arrival.

A number of diagnostic tests for SARS-CoV-2 infection (e.g., NAATs, antigen tests) have received emergency use authorizations (EUAs) issued by the Food and Drug Administration (FDA),1 but no diagnostic test has been approved by the FDA.

While testing nasopharyngeal specimens remains the current standard for diagnosing SARS-CoV-2, nasal (anterior nares or mid-turbinate) or oropharyngeal swabs are acceptable alternatives.2 Although lower respiratory tract samples have a higher yield than upper tract samples, they are often not obtained because of concerns about aerosolization of the virus during sample collection procedures. Some tests that have received EUAs can also be performed on saliva specimens. Other sample types, including stool samples, are currently being studied.

Some tests that have received EUAs allow for self-collection of specimens at home, but these specimens must be sent to a laboratory for processing. In addition, some tests allow for collection and testing of specimens by trained personnel in nonclinical settings, such as in the home or in nursing or assisted living facilities. This allows real-time antigen results to be obtained on site.

Nucleic Acid Amplification Testing for SARS-CoV-2 Infection

Reverse transcriptase polymerase chain reaction (RT-PCR)-based diagnostic tests (which detect viral nucleic acids) are considered the gold standard for detecting current SARS-CoV-2 infection. More recently, NAATs have included a variety of additional platforms (e.g., real-time loop mediated isothermal amplification [RT-LAMP]). Clinically, there may be a window period of up to 5 days after exposure before viral nucleic acids can be detected. However, false negative NAAT results can also occur outside of this 5-day window. Therefore, a single negative test result does not completely exclude SARS-CoV-2 infection in people with a high likelihood of infection based on their exposure history and/or their clinical presentation, and repeat testing using a NAAT should be considered.3

Antigen Testing for SARS-CoV-2 Infection

Antigen-based diagnostic tests (which detect viral antigens) are less sensitive than RT-PCR-based tests, but they have similarly high specificity when compared to RT-PCR-based tests. Antigen tests perform best early in the course of symptomatic SARS-CoV-2 infection, when the viral load is thought to be highest. When a person who is strongly suspected of having SARS-CoV-2 infection receives a negative result on an initial antigen test, repeat testing using a NAAT should be considered. Advantages of antigen-based tests are their low cost and rapid turnaround. The availability of immediate results makes them an attractive option for point-of-care tests in high-risk congregate settings where preventing transmission is critical. Antigen-based tests also allow for repeat testing to quickly identify persons with SARS-CoV-2 infection. Currently, there are limited data to guide the use of rapid antigen tests to detect or exclude SARS-CoV-2 infection in asymptomatic persons or to determine whether a person who was previously confirmed to have SARS-CoV-2 infection is still infectious.4

Serologic or Antibody Testing for Diagnosis of SARS-CoV-2 Infection

Unlike NAATs and antigen tests for SARS-CoV-2 that detect the presence of the virus, serologic or antibody tests are intended to identify persons with recent or prior SARS-CoV-2 infection. Because it may take 21 days or longer after symptom onset for seroconversion or detection of immunoglobulin (Ig) M and/or IgG antibodies to SARS-CoV-2,5-10 the COVID-19 Treatment Guidelines Panel (the Panel) does not recommend the use of serologic testing as the sole basis for diagnosing acute SARS-CoV-2 infection (AIII). Given that NAATs and antigen tests for SARS-CoV-2 occasionally yield false negative results, serologic tests have been used in some settings as an additional diagnostic test for patients who are strongly suspected to have SARS-CoV-2 infection. Using serology in combination with a NAAT to detect IgG or total antibodies 3 to 4 weeks after symptom onset maximizes the sensitivity and specificity to detect past infection.

No serologic tests for SARS-CoV-2 are approved by the FDA; some, but not all, commercially available serologic tests for SARS-CoV-2 have received EUAs issued by the FDA.1 Several professional societies and federal agencies, including the Infectious Diseases Society of America, the Centers for Disease Control and Prevention, and the FDA, provide guidance for clinicians regarding the use of serologic testing for SARS-CoV-2.

Several factors should be considered when using these tests, including:

  • Important performance characteristics, including the sensitivity and specificity (i.e., the rates of true positive and true negative results) of many of the commercially available serologic tests, have not been fully characterized. Serologic assays that have FDA EUAs should be used for public health and clinical use. Formal comparisons of serologic tests are in progress.
  • Serologic assays may detect IgM, IgG, IgA, and/or total antibodies, or a combination of IgM and IgG antibodies. Serologic assays that detect IgG and total antibodies have higher specificity to detect past infection than assays that detect IgM and/or IgA antibodies or a combination of IgM and IgG antibodies.
  • False positive test results may occur due to cross-reactivity from pre-existing antibodies to other coronaviruses.

Serologic Testing and Immunity to SARS-CoV-2 Infection

The Panel recommends against the use of serologic testing to determine whether a person is immune to SARS-CoV-2 infection (AIII). If serologic tests are performed and antibodies are detected, results should be interpreted with caution for the following reasons:

  • It is currently unclear how long antibodies persist following infection; and
  • It is currently unclear whether the presence of antibodies confers protective immunity against future infection.

In communities where the prevalence of SARS-CoV-2 infection is low, the proportion of positive tests that are false positives may be quite high. In these situations, confirmatory testing using a second independent antibody assay, ideally one that uses a different antigenic target (e.g., the nucleocapsid phosphoprotein if the first assay targeted the spike glycoprotein), can substantially improve the probability that persons with positive test results are antibody positive.

Assuming that the test is reliable, serologic tests that identify recent or prior SARS-CoV-2 infection may be used to:

  • Determine who may be eligible to donate convalescent plasma.
  • Measure the immune response in SARS-CoV-2 vaccine studies.
  • Estimate the proportion of the population exposed to SARS-CoV-2.

Serologic tests should not be used to:

  • Make decisions about how to group persons who are residing in or being admitted to congregate settings (e.g., schools, dormitories, correctional facilities); or
  • Determine whether persons should return to the workplace.

References

  1. Food and Drug Administration. Coronavirus disease 2019 (COVID-19) emergency use authorizations for medical devices. 2020. Available at: https://www.fda.gov/medical-devices/emergency-situations-medical-devices/emergency-use-authorizations#covid19ivd. Accessed October 20, 2020.
  2. Centers for Disease Control and Prevention. Interim guidelines for collecting, handling, and testing clinical specimens from persons for coronavirus disease 2019 (COVID-19). 2020 Available at: https://www.cdc.gov/coronavirus/2019-ncov/lab/guidelines-clinical-specimens.html. Accessed October 20, 2020.
  3. Kucirka LM, Lauer SA, Laeyendecker O, Boon D, Lessler J. Variation in false-negative rate of reverse transcriptase polymerase chain reaction-based SARS-CoV-2 tests by time since exposure. Ann Intern Med. 2020;173(4):262-267. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32422057.
  4. Centers for Disease Control and Prevention. Coronavirus Disease 2019 (COVID-19): interim guidance for rapid antigen testing for SARS-CoV-2. 2020 Available at: https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html. Accessed October 15, 2020.
  5. Guo L, Ren L, Yang S, et al. Profiling early humoral response to diagnose novel coronavirus disease (COVID-19). Clin Infect Dis. 2020;71(15):778-785. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32198501.
  6. Haveri A, Smura T, Kuivanen S, et al. Serological and molecular findings during SARS-CoV-2 infection: the first case study in Finland, January to February 2020. Euro Surveill. 2020;25(11). Available at: https://www.ncbi.nlm.nih.gov/pubmed/32209163.
  7. Long QX, Liu BZ, Deng HJ, et al. Antibody responses to SARS-CoV-2 in patients with COVID-19. Nat Med. 2020 ;26(6):845-848. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32350462.
  8. Okba NMA, Müller MA, Li W, et al. SARS-CoV-2 specific antibody responses in COVID-19 patients. medRxiv. 2020;Preprint. Available at: https://www.medrxiv.org/content/medrxiv/early/2020/03/20/2020.03.18.20038059.full.pdf.
  9. Xiang F, Wang X, He X, et al. Antibody detection and dynamic characteristics in patients with COVID-19. Clin Infect Dis. 2020; Published online ahead of print. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32306047.
  10. Zhao J, Yuan Q, Wang H, et al. Antibody responses to SARS-CoV-2 in patients of novel coronavirus disease 2019. Clin Infect Dis. 2020; Published online ahead of print. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32221519.