SARS-CoV-2 antibodies: 3 classes of tests...which is most useful?
by James Rudge, PhD, Technical Director, Neoteryx on Jun 5, 2020 9:15:00 AM
The scientific and medical communities continue investigating potential vaccines and therapies against SARS-CoV-2, or severe acute respiratory syndrome coronavirus 2, the novel coronavirus that causes COVID-19 illness. Many believe that testing for the presence of SARS-CoV-2 raised antibodies is one of the best ways to gain an understanding of a possible full or partial immunity to the disease. Furthermore antibody testing will impart an understanding of which interventions will be most effective in preventing and treating these infections.
Throughout the Spring of 2020, we have received equal amounts of insightful information and misinformation about SARS-CoV-2 antibody testing. Conflicting news reports about exactly what antibody testing can—and can’t—tell us has led to widespread frustration and confusion.
Some serology studies have been launched in an effort to track the presence of antibodies in blood samples from people who have already been exposed to the coronavirus or recovered from a COVID-19 infection. Will these “serology surveys” provide us with definitive information about herd immunity and insights on how long immunity might last? We’re not yet sure.
Taking a Closer Look at SARS-CoV-2 Antibody Testing
To understand why antibody testing doesn’t always lead to definitive answers about a disease, or about immunity to that disease, let’s take a closer look at how antibody testing works.
When we talk about testing for antibodies of SARS-CoV-2, we are actually talking about three classes of antibodies: IgA, M and G. Among scientists at the forefront of COVID-19 research, it seems that M and G are the most popular classes for testing right now.
Currently, there are tests, either recently launched or in the process of being developed, that can detect antibodies A,M and G, or M & G, or just G. However, it is important to note that all SARS-CoV-2 antibody tests are not going to be as good as one another. Some will be more sensitive, some will be more specific, and some will be both with little to no false positives or negatives. What does this mean? To illustrate, let’s examine an ELISA test, or enzyme-linked immunoassay. An ELISA is a laboratory test that detects antibodies in blood. In the case of SARS-CoV-2, the antibodies which are being are investigated are being raised against the spike protein and, in some cases, the nucleocapsid.
A lab that is conducting SARS-CoV-2 antibody testing might use an ELISA test, which simultaneously looks for M and G, but can’t distinguish between the two. The test simply indicates that a person has been exposed to the coronavirus and has developed some kind of immune response. It does not say if the individual has developed IgG antibodies that can possibly lead to an individual developing an immunity, which has been proven to occur for other viral infections. Moreover, there are ELISAs that look for one type of antibody, usually IgG, which indeed tells us if a person has developed possible immunity.
There are also multiplex assays that have the potential to look for all three classes of antibodies, and even various mutations of the spike protein (the main immunoassay target) of the novel coronavirus. However, it is worth noting that the processes and methods involved can vary widely—even the lateral flow devices differ. There are some that can measure both M and G. Among all these techniques, IgG is far and away the most popular.
Many labs are now optimized for antibody testing using remote blood samples collected with the Mitra® device, which is based on volumetric absorptive microsampling, or VAMS® technology. The VAMS technology allows people to collect a capillary blood sample in the safety of their homes and the dried sample is then sent a laboratory in the regular post. Moreover, the VAMS samples can be measured on the above-mentioned immunoassay platforms regardless of which class of immunoglobulins are being measured. Scientists are still refining their testing methods. Some assays will give more false positives or negatives than others, which will define the intended market for the test.
For additional information on immunoassays and extraction methods using Mitra and VAMS for SARS-CoV-2 testing, please see our “Tech Brief: A Review of Extraction Methods Using Mitra from the Current Literature.”
This chart below, which appeared in an article titled “Why Do We Need Antibody Tests for COVID-19 and How to Interpret Test Results” from Diazyme Laboratories, is for illustrative purposes only. It shows variation of levels of SARS-CoV-2 RNA and Antigen IgM and IgG after infection.