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SARS-CoV-2 Serology (COVID-19) Antibody (IgG), Immunoassay

Test code(s) 39504

Question 1. What types of immunoassays are available for SARS-CoV-2 (COVID-19)?  

Currently, 2 types of serological assays are available in the marketplace for SARS-CoV-2 testing:

  1. Laboratory-based immunoassays: There are various types of immunoassays in this category, such as enzyme-linked immunosorbent assays (ELISAs), chemiluminescent microparticle immunoassays (CMIAs), and immunometric assays. These tests can be qualitative or quantitative and are generally performed using serum. These immunoassays use a solid phase coated with viral antigen to bind SARS-CoV-2 antibodies, but different tests may use different solid phases. For example, ELISAs use a plate and CMIAs use paramagnetic microparticles. Quest Diagnostics currently offers a high-complexity laboratory-based qualitative immunoglobulin G (IgG) SARS-CoV-2 immunoassay and not a rapid diagnostic test (RDT). Our IgG testing is also amenable to high-throughput testing commensurate with national pandemic testing needs. 
  2. RDTs: These are typically lateral flow assays that can be used for point-of-care (POC) testing. RDTs most frequently test for SARS-CoV-2 IgG, immunoglobulin M (IgM), or viral antigen. These tests usually use blood samples from a finger stick, and some can use saliva or other specimen types.1,2 Quest does not offer RDTs for SARS-CoV-2 antibody or antigen testing in its regional laboratories.

Question 2. What is the US Food and Drug Administration (FDA) policy for commercial manufacturers and laboratory development/use of serology tests for SARS-CoV-2?

The FDA’s first guidance to the developers of serology tests for SARS-CoV-2 was more flexible than for molecular tests. On May 4, 2020 the FDA issued a revision to this policy with the following expectations for antibody test developers:

  • To submit requests for emergency use authorization (EUA),with their validation data, within 10 business days from the date they notified the FDA of their validation testing OR from the date of this policy, whichever is later.   
  • The FDA provided specific performance threshold recommendations for specificity and sensitivity.2

SARS-CoV-2 IgG antibody assays offered by Quest Diagnostics have been authorized through an EUA process.3-5 The FDA has created a website to summarize the performance of the serology tests that have been granted EUA.6

The FDA also issued a letter for healthcare providers on the use of serological (antibody) tests for COVID-19 on April 17, recommending continued use of serological tests, as appropriate, with awareness of their limitations.7 Refer to Question 9 for more information about cross-reactivity due to past or present infection with other coronaviruses.

Question 3. How does Quest Diagnostics ensure that the serologic assays are accurate?    

All SARS-CoV-2 IgG antibody assay kits used by Quest Diagnostics have been authorized through an EUA process. Quest Diagnostics also ensures that tests offered for SARS-CoV-2 IgG are highly specific and have validated accuracy:

  1. We use laboratory-based immunoassays from manufacturers who have demonstrated robust validation of their kits. Highlights of the manufacturers’ validation include:
    1. Clinical performance of approximately 90% to 100% (assessed as percent agreement of serology results on known COVID-19 PCR positive cases).
    2. Specificity of approximately 99% to 100%. This was assessed by performing cross-reactivity studies utilizing serum samples positive for antibodies to other respiratory viruses, as well as panels of samples from pre-COVID times (2010, 2017, and 2019).3-5
  2. Before starting patient testing, we verify the performance characteristics of the kits by doing CLIA/CAP-required in-laboratory validations using stringent acceptability criteria for precision, reproducibility, accuracy, method comparison, cross-reactivity, and clinical performance.

Question 4. How soon do IgG antibodies to SARS-CoV-2 appear in a patient who has been exposed to the virus?

The antibody response to SARS-CoV-2 usually starts with IgM and/or IgA being detected first, followed by the longer-lasting and more-specific IgG. Data suggest that IgG antibodies can be detected from 10 days after SARS-CoV-2 exposure or post symptom onset. However, some people do not generate detectable IgG antibodies after infection, because of an underlying immune disorder, immunosuppression, or other reasons. Additionally, an individual immune response can vary in the speed and strength of IgG production based on infective dose or viral burden upon exposure to SARS-CoV-2.   

Question 5. Can antibody tests be used to diagnose SARS-CoV-2 infection?

No. At present, no antibody tests have an intended use that includes definitive diagnosis of, or ruling out of, current SARS-CoV-2 infection. Serological IgG assays provide information about whether a person has been exposed to SARS-CoV-2. A molecular diagnostic test should be considered to diagnose or rule out current infection.2

Question 6. How do I interpret SARS-CoV-2 Serology (COVID-19) Antibody (IgG), Immunoassay results?

Results from this qualitative test for SARS-CoV-2 IgG can be positive (reactive), negative (non-reactive), or, occasionally, equivocal (borderline).

A positive (reactive) result indicates detection of SARS-CoV-2 IgG, which may suggest exposure to SARS-CoV-2. It usually takes at least 10 days after symptom onset for IgG to reach detectable levels. Although the relationship between IgG positivity and immunity to SARS-CoV-2 has not yet been established, the detection of IgG antibodies may suggest an immune response to SARS-CoV-2 (COVID-19) after resolution of infection with SARS-CoV-2. Antibody tests should not be used to diagnose SARS-CoV-2 infection. Patients with symptoms should be evaluated with a molecular assay instead.

A negative (non-reactive) result indicates that SARS-CoV-2 IgG is not present at a level that is detectable by the SARS-CoV-2 Serology (COVID-19) Antibody (IgG), Immunoassay. It usually takes at least 10 days after symptom onset for IgG to reach detectable levels and may take up to 3 weeks for a full antibody response to develop. The rate of IgG development can vary between individuals. The IgG response is usually slower in people who are either on immunosuppressive therapies or are immunocompromised due to a variety of health conditions. Therefore, some people may take even longer than 3 weeks to develop SARS-CoV-2 IgG antibodies and some people may not develop antibodies at all. A repeat serology IgG test should be considered after an additional 2- to -3-week period for patients who are initially antibody negative and are thought to have clinically recovered from a SARS-CoV-2 infection. It is currently not known for how long IgG remains detectable after exposure to SARS-CoV-2.

Negative results suggest a person has not been exposed to SARS-CoV-2 or has been exposed very recently (antibodies have not yet been produced) and is at risk for infection, particularly in those who have been in contact with the SARS-CoV-2 virus. Follow-up testing with a molecular diagnostic assay should be considered, if clinically indicated.

An equivocal (borderline) result indicates that IgG was detected at a level close to the threshold of the limit of detection for the test. Equivocal results may represent an early infection, detection of an IgG generated from a past infection (refer to Question 9 for cross-reactivity with non-SARS-CoV-2 coronavirus strains), or, in some cases, an underlying immune disorder, immunosuppression, or other reasons.

In populations with a high prevalence of non-SARS-CoV-2 coronavirus strains, a positive result could also be due to past or present infection with one of these strains (refer to Question 9).

Question 7. Does a positive result for IgG to SARS-CoV-2 mean that the patient is immune? 

Presence of IgG to SARS-CoV-2 indicates that the patient has mounted an immune response to the virus. Although the immune response may protect against reinfection, this has yet to be established. An animal study of SARS CoV-2 demonstrated protective immunity after infection.7 Early human study demonstrated development of protective neutralizing antibody responses in individuals infected with SARS-CoV-2.8 It is also not known how long antibodies to the virus will protect someone, if at all. Scientists are conducting research to answer these questions. The persistence of detectable IgG antibodies and neutralizing viral antibodies in SARS-CoV patients for up to 720 days suggests that past infection may protect from recurrent SARS-CoV infection for up to 2 years.9 Whether past SARS-CoV-2 provides such protection has not been established.

Question 8. What are the indications for use of the SARS-CoV-2 Serology (COVID-19) Antibody (IgG), Immunoassay?

Testing for SARS-CoV-2 IgG, including for individuals who may be asymptomatic or are ≥10 days after SARS-CoV-2 exposure or post-symptom onset, can play a critical role in the fight against COVID-19. SARS-CoV-2 IgG testing can be used to2,10,11

  • Identify SARS-CoV-2 exposed persons with PCR-negative results, especially for patients who present late with a very low viral load below the detection limit of RT-PCR assays, or when lower respiratory tract sampling is not possible.
  • Identify whether people have been exposed to SARS-CoV-2 and have mounted an immune response.
  • Assess how many people have been exposed to SARS-CoV-2 in a population, by identifying individuals who have developed antibodies to the virus.
  • Possibly help identify individuals who may be able to donate convalescent plasma as a possible treatment for those who are seriously ill from COVID-19.

In the future, this test, along with other clinical data, may help identify individuals who may be less susceptible to infection and can return to work.

Question 9. Is there cross-reactivity between SARS-CoV-2 antibodies and any of the human coronaviruses or SARS-CoV?

Specificity of the SARS-CoV-2 IgG immunoassays (ELISA and CMIA) is approximately 99%. Manufacturers of the tests that Quest provides tested panels of samples from pre-COVID times (2010, 2017, and 2019), with less than 1% of samples showing positive results.3-5 These findings suggest that antibodies against SARS-CoV and other coronaviruses are not commonly found in the general public. 

Question 10. What is the role of neutralization assays, and are they available to assess immunity to SARS-CoV-2?

Neutralization assays are done to help assess the ability of neutralizing antibodies (NAbs) in patient serum to neutralize virus infectivity. These tests are useful to assess whether past infection (or vaccination, if available) has provided protection against infection. In these tests, serial dilutions of patient serum are spiked with known virus concentrations and then added to cell lines. After incubation, infected cells are quantified to determine the effectiveness of patient antibodies in neutralizing the virus.1 The presence of NAbs may suggest immunity, and convalescent serum with NAbs is being studied as a treatment for COVID-19. Testing for NAbs is only available at some public health laboratories and research facilities. It is not performed at Quest Diagnostics.


  1. Kobokovich A, West R, Gronvall G. Serology based tests for COVID-19. Center for Health Security. Accessed April 12, 2020. https://www.centerforhealthsecurity.org/resources/COVID-19/serology/Serology-based-tests-for-COVID-19.html
  2. FAQs on testing for SARS CoV-2. US Food and Drug Administration. Accessed April 12, 2020. https://www.fda.gov/medical-devices/emergency-situations-medical-devices/faqs-diagnostic-testing-sars-cov-2
  3. Anti-SARS-CoV-2 ELISA (IgG). Instructions for use. Euroimmun; March 2020.
  4. Abbott Architect SARS-CoV-2 IgG. Package insert. Abbott; April 2020.
  5. VITROS Immunodiagnostic Products Anti-SARS-CoV-2 IgG. Instructions for use. Ortho-Clinical Diagnostics; April 2020.
  6. US Food and Drug Administration. EUA authorized serology test performance. Accessed May 7, 2020. https://www.fda.gov/medical-devices/emergency-situations-medical-devices/eua-authorized-serology-test-performance
  7. Bao L, Deng W, Gao H, et al. Reinfection could not occur in SARS-CoV-2 infected rhesus macaques. bioRxiv. 2020. doi:10.1101/2020.03.13.990226
  8. Wu F, Wang A, Liu M, et al. Neutralizing antibody responses to SARS-CoV-2 in a COVID-19 recovered patient cohort and their implications. medRxiv. 2020. doi:10.1101/2020.03.30.20047365
  9. Mo H, Zeng G, Ren X, et al. Longitudinal profile of antibodies against SARS-coronavirus in SARS patients and their clinical significance. Respirology. 2006;11(1):49–53. doi:10.1111/j.1440-1843.2006.00783.x
  10. Important information on the use of serological (antibody) tests for COVID-19–letter to health care providers. US Food and Drug Administration. Updated April 17, 2020. Accessed April 23, 2020. https://www.fda.gov/medical-devices/letters-health-care-providers/important-information-use-serological-antibody-tests-covid-19-letter-health-care-providers
  11. Infectious Diseases Society of America. IDSA COVID-19 antibody testing primer. Updated April 22, 2020. Accessed April 23, 2020. https://www.idsociety.org/globalassets/idsa/public-health/covid-19/idsa-covid-19-antibody-testing-primer.pdf
This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
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