What to know before getting a COVID-19 antibody test

What to know before getting a COVID-19 antibody test
(Source: WVIR)

(CBS News) - In March, as the United States faced a shortage of COVID-19 diagnostic tests to determine who was currently infected with the virus, the Food and Drug Administration began allowing antibody tests into the country without FDA review or formal clearance.

A COVID-19 molecular diagnostic test, sometimes referred to as a virology test, is often conducted through a nasal or throat swab. It is designed to detect if you presently have the COVID-19 virus.

A COVID-19 antibody test, or serology test, usually involves a blood sample and determines the presence of antibodies, which may signal that you previously had the viral infection.

"The two different types of tests give us complementary information about the pandemic," said Dr. Alex Marson, an infectious disease specialist and the director of the Gladstone UCSF Institute for Genomic Immunology. "The one tells us about who's actively infected. The other tells us who has been infected."

Dr. Marson and Dr. Patrick Hsu, an assistant professor of bioengineering at the University of California, Berkeley both told 60 Minutes it is presently unknown if a positive antibody test means protection from future infection.

"We do not know if a positive antibody test means that you have protective immunity," said Dr. Hsu. "It will certainly mean that in some cases, but how much, and for how long, and how many times? We do not yet know. People should not assume that, if you have a positive antibody test that you're immune, [that] you don't need to wear a mask, and [that] you don't need to socially distance."

A small sample size study from China published on June 18 in Nature Medicine found antibodies could fade in as soon as two to three months after the time of infection.

Both Dr. Marson and Dr. Hsu were interviewed by 60 Minutes correspondent Sharyn Alfonsi as part of a three-month investigation into the accuracy of antibody tests available in the U.S. The physicians assembled a team of scientists to study the accuracy of COVID-19 antibody tests in early March.

"I will say that the best tests, especially tests where blood samples are being sent off to labs, seem to have responsible test performance characteristics," Dr. Marson told Alfonsi. "Does that mean that they're perfect? No"

The FDA says the prevalence of COVID-19 in a specific area and amongst different groups of people will factor into the likelihood of false positive or false negative results.

"I think when you ever get a positive test, it's important to undergo confirmatory testing, whether with a different antibody test, or an independent test that looks for a different viral antigen, or a different antibody isotype," Dr. Hsu told 60 Minutes. "This will be a way that we can guard against false positives. The statistical likelihood that you would test false positive twice is generally quite unlikely if you use independent tests."

Getting multiple independent tests might be more difficult than it sounds.

Quest Diagnostics and LabCorp are two of the largest laboratory testing providers in the U.S. They have collectively processed more than 3.5 million COVID-19 serology tests.

Quest told 60 Minutes it presently offers antibody tests manufactured by Abbott Laboratories, EUROIMMUN AG, and Ortho Clinical Diagnostics. LabCorp said it currently uses serology tests made by Abbott and Roche.

Both lab companies told 60 Minutes they do not allow patients to choose which antibody test is used ahead of a sample being processed.

Quest said the testing platform used to process a sample depends on a lab's volume and capacity. The company said it can run 200,000 serology tests per day and has conducted 2.1 million since April.

LabCorp said it began serology testing on a limited basis in March and now has the ability to run 300,000 samples a day. It does not disclose the daily volume of antibody testing, but told 60 Minutes it has processed approximately 1.4 million tests so far.

As of June 27, the Centers for Disease Control and Prevention reported more than 32 million tests have been processed in the U.S. with a positivity rate of 9%. The CDC declined to tell 60 Minutes the specific sources of its aggregated data. A spokesperson told 60 Minutes the 32 million tests include both viral and antibody test results. The CDC also says not all test results are reported to them.

A Government Accountability Office report released on June 25, 2020 says the watchdog agency found that the CDC has made mistakes in its collection of data about COVID-19 testing. It says the CDC was gathering data about testing that combined antibody tests with diagnostic tests in their overall testing numbers, a practice that has been criticized by scientists as mixing apples and oranges. The CDC made an effort to correct this practice and advise states to separate virology and serology test results, but the overall testing numbers from prior to June may still be inaccurate. Given the problems with accuracy of some antibody tests, it is also unclear whether all antibody positives should be considered true positives.