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The US Food and Drug Administration (FDA) has authorized the first two COVID-19 blood tests that generate an estimate of the number of antibodies present in a person’s blood.
Both “semi-quantitative” tests, developed by Siemens Healthineers — the ADVIA Centaur COV2G and Atellica IM COV2G — give only estimates, not precise measurements of the number of antibodies produced by SARS-CoV-2.
“With this numerical value, clinicians will have a baseline and be better equipped to assess changes of an individual’s immune response to the SARS-CoV-2 virus,” Siemens says on its website.
The sensitivity for both tests is 100% with specificity above 99.8%, according to the company.
Tests Useful, but…
Catherine Troisi, PhD, infectious disease epidemiologist with UTHealth School of Public Health in Houston, Texas, told Medscape Medical News that the tests will add information because the previous antibody tests, licensed under an emergency use authorization, have simply indicated whether a person has the antibodies or not, not how many they have.
The tests will be useful, she said, but cautioned that when you’re infected with a virus, your body makes lots of different types of antibodies.
The tests won’t show whether the antibodies are the “neutralizing” antibodies needed for protection from a subsequent infection, Troisi said.
Additionally, she said it’s important to remember there are two arms of the immune system: B cells and T cells.
“The B cells make antibodies and that’s what we can easily measure in a test, but it may be that the T-cell arm is more important with these viruses and immunity. There’s a little bit of data out there that says that might be the case,” Troisi said.
She added that much more research needs to be done on the role of antibodies and how long immune response lasts.
Medscape Medical News has previously reported that antibody levels in patients with mild COVID-19 — the level of disease most people have — appear to drop by half within 36 days, raising questions about their effect on immunity.
Medscape also reported on results of a study from China in Nature Medicine showing that, 8 weeks after recovery, antibody levels fell to undetectable levels in 40% of asymptomatic people and 13% of symptomatic people.
Tim Stenzel, MD, PhD, director of the Office of In Vitro Diagnostics and Radiological Health at FDA, acknowledged in the July 31 press release the unknowns for antibodies concerning immunity.
However, he said, “Being able to measure a patient’s relative level of antibodies in response to a previous SARS-CoV-2 infection may be useful as we continue to learn more about the virus and what the existence of antibodies may mean.”
He added, “Patients should not interpret results as telling them they are immune, or have any level of immunity, from the virus.”
The FDA also cautions against patients using results from these tests or any blood tests as a reason to relax protection efforts against COVID-19.
Additionally, the FDA says, the tests should not be used to diagnose active infection as they don’t detect the virus itself, just the antibodies the immune system has developed in response to the virus.
Troisi has disclosed no relevant financial relationships.
Marcia Frellick is a freelance journalist based in Chicago. She has previously written for the Chicago Tribune and Nurse.com and was an editor at the Chicago Sun-Times, the Cincinnati Enquirer, and the St. Cloud (Minnesota) Times. Follow her on Twitter at @mfrellick