Bio-Rad Laboratories Inc. has launched its SARS-CoV-2 Total Ab test, which is a blood-based assay to identify all the antibodies that are developed by the human body in response to the novel coronavirus SARS-CoV-2. Companies have been racing to offer serological tests that work to detect the antibodies developed during a COVID-19 infection that remain present in the blood after the initial infection clears.

There are more than 120 serological tests available in the U.S. for SARS-CoV-2, which the FDA has allowed under relaxed guidelines but that have been found to have widely varying results. Only nine of them have received emergency use authorization (EUA) from the FDA; two of these are total antibody tests, of which Bio-Rad’s test is one. The company has already launched the test and is in conversations with the FDA around the EUA submission. Testing has found that its SARS-CoV-2 Total Ab test has 98% sensitivity and 99% specificity.

Accuracy matters

“Bio-Rad has deep experience, dating back to acquisition of Sanofi Pasteur in the late 1990s, in developing serological infectious disease assays,” Dara Wright, executive vice president and Clinical Diagnostics Group President at Bio-Rad told BioWorld. Bio-Rad acquired Pasteur Sanofi Diagnostics for about $210 million in 1999. “For the recent SARS-CoV-2 Total Ab test, we were able to get a jump start on development by leveraging some of the biological reagents from a SARS-CoV-1 assay we had developed during the 2003 outbreak and also partner with Institut Pasteur, in Paris, to draw from their deep expertise as well.”

Sensitivity is important in so far as the results are likely to be a “true positive,” which sets expectations for an individual patient on the likelihood their own disease has been detected. Specificity is the “true negative” rate. Very high rates for both are crucial for effective public health management of the ongoing pandemic, which is expected to rely on population studies known as serosurveys in determining potential group immunity to COVID-19 infection.

Once at least 60% of a population has had an infectious disease with accrued immunity, a concept known as “herd immunity,” the remainder of the group is thereby offered some protection from transmission. The earliest results from U.S. regional serosurveys have found widely varying infection rates from 4% in Santa Clara County, Calif. and 5% in Los Angeles County, Calif. to as high as 24.7% in New York City, according to New York governor Andrew Cuomo. The population sampling methodology has ranged widely in early studies, but the World Health Organization has published a standard investigation protocol for serosurveys.

The margin of error for serosurvey results could also vary widely depending largely on the accuracy of the underlying assays used. Systematic application of highly accurate serological testing is crucial to advancing the understanding of the rates of prior infection, as well as providing a basis to understand the potential immunity conferred and then to test how long it might provide effective protection.

Outside the U.S., countries that seem to be effectively managing their ongoing response, such as Germany, Sweden, China and Singapore, are starting to systematically implement serological testing to start to assess potential immunity. Within the U.S., the National Institutes of Health has started a national serological survey in 10,000 healthy adults who have no history of COVID-19 symptoms or a confirmed diagnosis of the disease.

Total test

Bio-Rad’s serological COVID-19 test is designed to assess all the antibodies generated by the disease. In particular, it tracks a trio of antibodies: Immunoglobulin G (IgG), Immunoglobulin M (IgM) and Immunoglobulin A (IgA). These are different classes of antibodies that come into play throughout the course of the disease – and tracking all three is expected to offer the most reliable results. These antibodies have been cross-reactivity tested to ensure that they do not result from non-CoV-2 coronaviruses, such as those that are associated with common colds.

“Different classes of antibodies (IgG, IgM, IgA) appear and become more prevalent at different time points post infection,” explained Wright. “Having a test that detects all serotypes enables earlier detection post infection. Immune response typically becomes detectable around Day 7, first with IgM, followed by IgG.”

“IgG is associated with sustained immunity, protection from reinfection is unknown,” she continued. “IgA is a secretory antibody that plays a role in mucous membrane protection; recent publications show that levels may correlate to disease severity and lung damage risk.”

Four of the SARS-COV-2 tests that have received an EUA from the FDA are for IgG only, while another three tests are for IgG and IgM. Only one “total” antibody test has received an EUA from the agency: the Vitros Anti-SARS-COV-2 Total test from Ortho-Clinical Diagnostics Inc. that received its authorization on April 14. It qualitatively measures all antibodies to the SARS-CoV-2 virus (IgG, IgM, IgA and other isotypes), which is comparable to what the Bio-Rad SARS-CoV-2 Total Ab test is designed to measure.

The Ortho-Clinical test is qualitative, meaning it can only describe the presence or absence of those antibodies but not the amount present. A quantitative antibody test could prove important in establishing if an individual has immunity and how long it is likely to last.

The Bio-Rad test is “semi-quantitative,” Wright said. That means the “relative abundance can be determined as compared to a standard.” But she cautioned that “the science is evolving as to the importance of quantitation.”

Validation

Based on almost 700 samples collected before the full-blown COVID-19 outbreak, the Bio-Rad SARS-CoV-2 Total Ab test was found to have 99.56% specificity. Sensitivity was determined by testing patients who had tested positive with a PCR analysis of a nasal swab; those patients were evaluated after eight days and the test was found to have 97.5% sensitivity.

The Bio-Rad SARS-CoV-2 Total Ab test was also validated in a longitudinal 51-patient study in COVID-19 patients in three Paris hospitals. All the patients were PCR-positive and had COVID-19 clinical symptoms. Out of a subset of 40 patients, 39 of them took more than eight days but less than 42 days after the onset of symptoms to be antibody-positive.

Wright noted that the ELISA kit format that the test relies upon is “exceptionally scalable.” She estimated that, depending on the demand, Bio-Rad has the capacity to perform millions of tests per week.

It will take further epidemiological and clinical research to understand precisely how COVID-19 antibodies may relate to the immunity that is typically conferred by acute viral infections. For now, the top priority in the U.S. is to ensure widely available, highly accurate serological testing.

When queried on how to circumnavigate the current U.S. testing difficulties, Wright responded that the focus needs to be on “establishing minimally acceptable performance criteria, especially for specificity – also enabling testing at scale for broad-based screening, especially for front-line workers and in urban centers.”

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