Antibody Detection

PLASMA OR SERUM ANTIBODY TESTING FOR COVID-19

JUNE 30, 2020 UPDATE

This review was prepared with significant input from Ernst J. Schaefer, MD, Professor of Medicine, Tufts University School of Medicine, Boston, MA, Chief Medical Officer & Laboratory Director, Boston Heart Diagnostics, Framingham, MA.

Antibody test for coronavirus in a laboratory (JENS MAYER-Associated Press)

Background

After exposure to an infectious agent, the human immune system recognizes specific components (called antigens) of that pathogen as foreign. These antigens are targeted by the immune system which responds by making antibodies against the antigens. In general, after viral infections IgM and IgA antibodies levels rise in the first 3-7 days following infection and then gradually decline, while IgG antibodies start increasing after 12-17 days and then persist for a fairly long period of time. Antibody levels are useful to document exposure and potential immunity. These tests are often referred to as serological tests, and the development of antibodies measurable in serum is referred to as seroconversion. It is important to understand that serological or antibody tests do not detect the virus itself, but the antibodies made by the immune system. To detect SARS CoV-2 virus, we use RT-PCR tests (see virology on this website).

Different systems exist for quantifying antibody levels. Positive tests for IgG, IgM, and IgA are generally described as > 1.0 arbitrary unit/mL or AU/mL. Values for IgG antibodies > 6.5 AU/mL correspond to an antibody titer of approximately > 1:320 and IgG values > 20 AU/mL correspond to titers > 1:1000. An IgG antibody titer of > 1:320 has been recommended by the FDA as a threshold for determining if convalescent plasma should be used in a patient.

Availability of Antibody Testing for COVID-19

The most commonly used test to detect antibodies is the Enzyme-linked immunosorbent assay (ELISA) which detects antibodies in the tested serum specimen that bind to the antigen of interest.

The website of 360dx.com, a business unit of Crain Communications, provides available diagnostic tests, both serological for antibodies and RT-PCR for virus detection, updated on a regular basis.

However, only some serological tests (listed in the Table) have received emergency use authorization by the FDA. Most assays measure total antibody concentrations, which correlate with levels of neutralizing anti-SARS-CoV-2 antibodies. The data suggest that IgG antibodies are most important for providing significant infection resolution. Antigens for the antibodies tested are various forms of the spike or S protein as well as nucleocapsid on the N protein.1 2 The sensitivity and specificity of these antibody assays for detecting negative and positive patients is best when using the combination of IgG and IgM antibodies and then often exceeds 95%.

The expected performance of the tests with their corresponding fact sheets are summarized in the table below. Please note that we do not currently know the prevalence of SARS-CoV-2 antibody positive individuals in the U.S. population, and prevalence may change based on the duration the virus is in the country and the effectiveness of mitigations. Thus, a prevalence of 5% was assumed for PPV and NPV calculations.

Seroconversion

Seroconversion, or the production of antibodies, has been observed to occur within 1 week of symptom onset for IgM antibodies and generally within 2 weeks of symptom onset for IgG antibodies.3 4 Available data indicate that quite high titers of SARS-CoV-2 IgG antibodies are found in a majority of convalescent patients if units of plasma are collected ≥14 days after resolution of symptoms. IgG antibody levels should be measured in donors who have become SARS-CoV-2 negative and have been asymptomatic for at least 14 days to assure adequate titers of IgG antibody in the donated plasma. These recommendations are in line with FDA guidance on use of convalescent plasma and requirements for receiving investigational new drug (or therapy) approval.5 As of June 30th, The FDA has provided twenty five tests with emergency use authorizations (EUAs) to detect COVID-19 antibodies (Table).

COVID-19 Serology Tests Performance Measures

COVID-19 Serologic Test Positive Predictive Value Calculator
FDA EUA authorized COVID-19 Serology Tests Table

References

  1. Cao W, Liu X, Bai T, Fan H, Hong K, Song H, et al. High-dose intravenous immunoglobulin as a therapeutic option for deteriorating patients with coronavirus disease 2019. Open Forum Infectious Diseases. 2020;7(3).
  2. Stadlbauer D, Amanat F, Chromikova V et al: SARS-CoV-2 Seroconversion in Humans: A Detailed Protocol for a Serological Assay, Antigen Production, and Test Setup. Curr Protoc Microbiol. 2020 Jun;57(1):e100. doi: 10.1002/cpmc.100.
  3. Okba NMA, Müller MA, Li W: Severe Acute Respiratory Syndrome Coronavirus 2-Specific Antibody Responses in Coronavirus Disease 2019 Patients. Emerg Infect Dis. 2020 Apr 8;26(7). doi: 10.3201/eid2607.200841. [Epub ahead of print].
  4. Guo L, Ren L, Yang S: Profiling Early Humoral Response to Diagnose Novel Coronavirus Disease (COVID-19). Clin Infect Dis. 2020 Mar 21. pii: ciaa310. doi: 10.1093/cid/ciaa310. [Epub ahead of print].
  5. CBER. Investigational COVID-19 Convalescent Plasma - Emergency INDs. https://www.fda.gov/vaccines-blood-biologics/investigational-new-drug-ind-or-device-exemption-ideprocess-cber/investigational-covid-19-convalescent-plasma-emergency-inds.

April 7, 2020

Coronavirus (COVID-19) Update: Serological Tests