COVID-19 Testing: CDC Guidance on Virus and Antibody Testing
NOTE: Information and guidelines may change rapidly. Check in with listed references in ‘Learn More – Primary Sources’ to best keep up to date.
SUMMARY:
The CDC has provided guidance on both viral testing for SARS-CoV-2 as well as the role of antibody testing. Testing for the presence of the virus during the pandemic remains the current diagnostic standard. While antibody testing can play a role for public health teams to understand the spread of the disease, currently its use as a diagnostic test for individuals remains limited. A COVID-19 vaccine will not affect the results of SARS-CoV-2 viral tests.
Viral Testing
Specimen Collection
- Obtain an upper respiratory specimen for initial diagnostic testing
- A nasopharyngeal (NP) specimen collected by a healthcare professional or
- An oropharyngeal (OP) specimen collected by a healthcare professional or
- A nasal mid-turbinate swab collected by a healthcare professional or by a supervised onsite self-collection (using a flocked tapered swab) or
- An anterior nares (nasal swab) specimen collected by a healthcare professional or by onsite or home self-collection (using a flocked or spun polyester swab) or
- Nasopharyngeal wash/aspirate or nasal wash/aspirate (NW) specimen collected by a healthcare professional
- Lower respiratory tract specimens
- Collect and test sputum in patients who develop a productive cough | Induction of sputum is not recommended
- Under certain clinical circumstances (e.g., those receiving invasive mechanical ventilation), a lower respiratory tract aspirate or bronchoalveolar lavage sample should be collected and tested as a lower respiratory tract specimen
How is SARS-CoV-2 RNA Testing Performed?
RT-PCR
- Usually performed using real-time reverse transcription polymerase chain reaction (RT-PCR)
- Qualitative detection of RNA
- Multiple tests on the market that can target various genes
- Envelope (env) | Nucleocapsid (N) | Spike (S) | RNA-dependent RNA polymerase (RdRp) | ORF1
- A positive test can only determine presence of SARS-CoV-2 RNA and not whether the virus is intact and capable of infecting others
Antigen
- Antigen tests can quickly detect fragments of proteins found on or within the virus by testing samples collected from the nasal cavity using swabs
- The benefit of antigen testing is speed, with results potentially available within minutes
- However, antigen tests, while very specific for the virus, are not as sensitive as molecular PCR tests
- Positive antigen results: Highly accurate but higher chance of false negatives | Negative antigen results may still need PCR confirmation prior to treatment decisions or to prevent inadvertent spread of SARS-CoV-2
Note: Prior receipt of a COVID-19 vaccine should not affect the results of SARS-CoV-2 viral tests (NAAT or antigen)
Breath Sample Analysis
- FDA has issued an emergency use authorization (EUA) for a diagnostic test that detects chemical compounds in breath samples associated with a SARS-CoV-2 infection
- Test is performed by a qualified, trained operator under the supervision of a health care provider licensed or authorized by state law to prescribe tests
- Results available in <3 minutes
Diagnostic Testing
Signs or Symptoms of COVID-19
- Positive test
- NAAT: Indicates infection regardless of vaccine status
- Positive antigen test result may need confirmatory testing if the person has a low likelihood of SARS-CoV-2 infection (e.g., no known exposure to a person with COVID-19 within the last 14 days or is fully vaccinated or has had a SARS-CoV-2 infection in the last 3 months)
- Isolate if positive test: Discontinue isolation 5 days after symptom onset and at least 24 hours after the resolution of any fever (without the use of fever-reducing medications) | Continue to wear mask around others for 5 additional days
- Some individuals may require extended isolation and precautions (e.g., severely immunocompromised)
- Testing is not recommended to determine when infection has resolved
- Loss of taste and smell may persist for weeks or months after recovery and need not delay the end of isolation
- Negative test
- If symptoms are consistent with COVID-19, may be a false negative | Isolation and further discussion with healthcare professional recommended
Testing to determine resolution of infection
- May be appropriate for severe illness or immunocompromise
- “For all others, a test-based strategy is no longer recommended except to discontinue isolation or precautions earlier than would occur under the symptom-based strategy”
Screening Testing
No Symptoms and No Close Contact with Someone Known to Have a COVID-19 Infection
- Asymptomatic or presymptomatic infection contribute to community SARS-CoV-2 transmission
- May help with re-opening of businesses, communities, and schools
- Point-of-care tests (e.g., antigen tests) can be particularly helpful due to short turn-around times
- Quarantine not required while results are pending
- Examples of screening programs
- Testing employees in a workplace setting
- Testing students, faculty, and staff in a school or university setting
- Testing a person before or after travel
How Early Will a Test Be Positive and How Long Until Negative?
- In patient with COVID-19 infection who tested positive using a nasopharyngeal swab
- Earliest detection: Day 1 of symptoms
- Peak levels highest within week 1 and therefore probability of detection will be highest during that time
- Viral load declines by week 3 and therefore virus more likely to be undetectable in to week 4
- Infection severity: More virus may be present in patients with severe disease and therefore it may take longer to obtain a negative test result vs someone with a mild COVID-19 infection
Performance of RT-PCR Viral Tests
- RT-PCR specificities are close to 100% because they target specific RNA sequences of the SARS-CoV-2 virus
- False negative results may be due to
- Inappropriate timing of collection vs symptom onset
- Poor sampling technique (need to sample at the back of the nose)
- False positive results may occur due to lab error or contamination
- However, even with good analytic performance, PPV and NPV are related to prevalence and therefore can differ between geographic regions
- In a setting with high COVID-19 prevalence, a negative test does not necessarily rule out the possibility that an individual is infected with SARS-CoV-2
Antibody Testing
General CDC Antibody Guidance
- According to the CDC
Antibody testing does not replace virologic testing and should not be used to establish the presence or absence of acute SARS-CoV-2 infection
Antibody testing is not currently recommended to assess for immunity to SARS-CoV-2 following COVID-19 vaccination, to assess the need for vaccination in an unvaccinated person, or to determine the need to quarantine after a close contact with someone who has COVID-19
Some antibody tests will not detect the antibodies generated by COVID-19 vaccines
Because these vaccines induce antibodies to specific viral protein targets, post-vaccination antibody test results will be negative in persons without history of previous infection, if the test used does not detect antibodies induced by the vaccine
- In general, antibodies will be detectable 7 to 14 days after illness onset and will be present in most people by 3 weeks
- Infectiousness likely decreased by that time
- Evidence suggests some degree of immunity will have developed
- IgM and IgG can appear together, usually within 1 to 3 weeks
- IgG antibodies appear to persist for at least several months
- Some individuals may be infected but will not develop antibodies
- Neutralizing antibodies can also be identified and are associated with immunity
- FDA requires companies providing antibody testing to obtain an EUA
What Are the Different Types of Antibody Tests?
- Antigenic Targets
- Spike glycoprotein (S): Present on viral surface and facilitates virus entry
- Nucleocapsid phosphoprotein (N): Immunodominant and interacts with RNA
- Protein targeting is important to reduce cross-reactivity (cause of false positives which may occur with other coronaviruses like the common cold) and improve specificity
- Types of Antibody Testing
- Binding antibody detection that use purified SARS-CoV-2 (not live virus)
- Point-of-care (POC) tests
- Laboratory tests that usually require skilled personnel and specialized equipment
- Neutralizing antibody detection (none currently FDA authorized)
- Serum or plasma is incubated with live virus followed by infection and incubation of cells
- Can take up to 5 days to complete the study
- Binding antibody detection that use purified SARS-CoV-2 (not live virus)
When Can Antibody Testing be Helpful?
Antibody testing may be helpful in the following situations
- Seroconversion: In a patient who did not receive a positive viral test
- A positive antibody test at least 7 days following acute illness onset but a previous negative antibody test may indicate new onset SARS-CoV-2 infection
- To support a diagnosis in the presence of a complex clinical situation, such as patients who present with COVID-19 complications (e.g., multisystem inflammatory syndrome and other post-acute sequelae of COVID-19)
- Note: Due to antibody persistence, a single positive antibody test result may reflect previous SARS-CoV-2 infection and not a recent illness
- Clinical, occupational health, and public health purposes, such as serologic surveys
Vaccination and Test Interpretation
- In a person never vaccinated
- testing positive for antibody against either N, S, or RBD indicates prior natural infection
- In a vaccinated person
- Testing positive for antibody against the vaccine antigen target, such as the S protein, and negative for other antigen: Suggests vaccine-induced antibody and not SARS-CoV-2 infection
- Testing positive for any antibody other than the vaccine-induced antibody, such as the N protein: Indicates resolving or resolved SARS-CoV-2 infection that could have occurred before or after vaccination
- The CDC states that
SARS-CoV-2 antibodies, particularly IgG antibodies, might persist for months and possibly years
Therefore, when antibody tests are used to support diagnosis of recent COVID-19, a single positive antibody test result could reflect previous SARS-CoV-2 infection or vaccination rather than the most recent illness
Learn More – Primary Sources:
Interim Guidelines for COVID-19 Antibody Testing in Clinical and Public Health Settings
CDC: Overview of Testing for SARS-CoV-2
Interpreting SARS-CoV-2 Test Results
The Promise and Peril of Antibody Testing for COVID-19
EUA Authorized Serology Test Performance
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