Interpreting COVID-19 antibody test results

7th Jan 2021

Nearly all immunocompetent individuals will develop an immune response of immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies to a SARS-CoV-2 infection. The most sensitive and earliest serological marker is total antibodies, IgM and IgG measured together. 

Immunoglobulin M and immunoglobulin G antibodies. SARS-CoV-2 virus. Illustration.

Figure 1. In response to SARS-CoV-2 infection, immunocompetent individuals will develop an immune response of immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies.

In most other infections, IgM antibodies develop before IgG. SARS-CoV-2 infections are somewhat unusual because IgM and IgG antibodies arise nearly simultaneously in the serum or, IgG might even appear before IgM.1

In SARS-CoV-2, an IgM and IgG enzyme-linked immunosorbent assay (ELISA) may become positive five to ten days after symptom onset, but much higher levels occur during the second and third week of illness. Fourteen days after symptom onset, over 90% of patients demonstrate seropositivity. Some individuals may take longer to develop antibodies depending on their immune status or may never develop antibodies if they are immunosuppressed. IgM falls to lower levels by week five and almost disappears by week seven but IgG persists beyond seven weeks.

Probability of detecting IgM and IgG antibodies after SARS-CoV-2 symptom onset. Graph.

Figure 2. In SARS-CoV-2, an immunoglobulin M (IgM) antibody and immunoglobulin G (IgG) antibody enzyme-linked immunosorbent assay (ELISA) may become positive five to ten days after symptom onset, but much higher levels occur during the second and third week of illness. IgM almost disappears by week seven while IgG persists beyond seven weeks (Adapted from Sethuraman, N, Jeremiah, SS, Ryo, A. 2020).

 

What are SARS-CoV-2 antibody tests used for?

Antibody tests can be used to measure the extent of COVID-19 infection in a community. Since the presence of antibodies indicates past or current infection, demographic and geographic patterns of serology test results can help determine which communities may have experienced a higher infection rate.

Remember that antibody tests are not useful for diagnosing an active COVID-19 infection. RT-PCR is the more appropriate test for this.

 

What does a positive SARS-CoV-2 antibody test mean?

Man with a positive SARS-CoV-2 antibody test. Illustration.

Figure 3. With a 95% specificity, false positives are a concern when using SARS-CoV-2 antibody tests. A false positive could indicate cross-reactivity, where antibodies to non-SARS human coronaviruses bind to the SARS-CoV-2 antigens used in the test. A true positive result indicates past SARS-CoV-2 infection. But, we are unsure if these antibodies will provide immunity to reinfection. Also, a positive result does not mean that the patient isn’t still shedding the virus or is no longer infectious. 

We know that a positive antibody test result indicates a past SARS-CoV-2 infection. However, a positive result could also be due to the detection of cross-reactivity, where antibodies to other coronavirus strains, which cause the common cold, may bind with the SARS-CoV-2 antigens used in the test. This would be a false positive test result.

It is estimated that 90% of persons age 50 and older have antibodies to the common non-SARS human coronaviruses. The low prevalence of COVID-19 in the general population and the low specificity of antibody tests can both contribute to a higher number of false-positive results. Initial studies suggest IgM and IgG ELISA tests only have a specificity of 95%, so false positive tests are a concern. 

We do not know yet if the antibodies that result from SARS-CoV-2 infection will provide immunity from getting infected again. In theory, seropositive individuals are expected to be at lower risk for reinfection compared to seronegative persons. If antibodies do provide immunity, we don’t know how much antibody is protective, or how long protection might last. Also, a positive result does not mean that the person is no longer shedding virus or is no longer infectious. 

 

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What does a negative SARS-CoV-2 antibody test mean?

 

Woman with negative a SARS-CoV-2 antibody test. Illustration.

Figure 4. A negative SARS-CoV-2 antibody test could mean no prior infection or that the antibodies had not developed yet because the test was done too early after infection or due to the patient’s immune status. False negatives could also occur if the antibody test has low sensitivity. 

A true negative result would occur if a patient never had a SARS-CoV-2 infection. However, a negative antibody test result does not rule out a SARS-CoV-2 infection. A negative result could also occur if the antibody test was done too early in the disease process for antibodies to have developed. It’s also important to note that this initial window when individuals may be serologically negative usually overlaps with when they are most infectious. 

A false negative result will occur if the antibody test has low sensitivity—that is, antibodies are present in the patient sample but not detected by the antibody test. 

Additionally, immunocompromised patients might have had SARS-CoV-2 but they did not develop antibodies to the virus so they would also get a negative antibody test result. One study found that only 95% of individuals with RT-PCR confirmed COVID-19 developed antibodies 20 days after symptom onset.2

A negative antibody test result should be followed with RT-PCR testing to rule out current SARS-CoV-2 infection if your clinical suspicion of COVID-19 is high.

That’s it for now. If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.

References

  1. Sethuraman, N, Jeremiah, SS, and Ryo A. 2020. Interpreting diagnostic tests for SARS-CoV-2. JAMA323: 2249–2251. PMID: 32374370
  2. Xu, X, Sun, J, Nie, S, et al. 2020. Seroprevalence of immunoglobulin M and G antibodies against SARS-CoV-2 in China. Nat Med26: 1193–1195. PMID: 32504052

Recommended reading

  • Association of Public Health Laboratories and Council of State and Territorial Epidemiologists. 2020. Public health considerations: serologic testing for COVID-19. APHL. 1https://www.aphl.org
  • Bryant, JE, Azman, AS, Ferrari, MJ, et al. 2020. Serology for SARS-CoV-2: apprehensions, opportunities, and the path forward. Sci Immunol5: eabc6347. PMID: 32530309
  • Infectious Diseases Society of America. 2020. IDSA COVID-19 antibody testing primer. https://www.idsociety.org
  • Theel, ES, Slev, P, Wheeler, S, et al. 2020. The role of antibody testing for SARS-CoV-2: is there one? J Clin Microbiol58: e00797–20. PMID: 32350074
  • Zhang, J, Gharizadeh, B, Lu, D, et al. 2020. Navigating the pandemic response life cycle: molecular diagnostics and immunoassays in the context of COVID-19 management. IEEE Rev Biomed EngPMID: 32356761
  • Zhang, N, Wang, L, Deng, X, et al. 2020. Recent advances in the detection of respiratory virus infection in humans. J Med Virol92: 408–417. PMID: 31944312