Fast virus test could be step closer
A small study by San Francisco researchers could bring the U. S. a step closer to having reliable and fast coronavirus diagnostic tests that generate results in minutes, instead of hours or days.
The study found that a new, rapid antigen test performed almost as well as stateoftheart tests, commonly referred to as PCR tests, at detecting positives among people who had high levels of the virus and were thus likely infectious. And the results come back to users much quicker, which could improve the coronavirus testing landscape.
The analysis was based on nasal specimens collected in September from 878 people at the 16th Street Mission BART Station in San Francisco’s Mission District.
The study is based on a small sample, and more studies are needed before this mode of testing can be deployed widely. It’s
unclear when that might happen. But the findings are notable because they suggest rapid antigen tests — though they are less sensitive than PCR tests — can be used to screen large numbers of people quickly, and appear to catch most infectious positive people, even if they don’t have symptoms. This is important because it would let people know they are positive at the time they’re infectious, so they can selfisolate and prevent further spread.
“It aims to answer the question, at that moment, are you likely to have a level of transmissible virus?” said Dr. Diane Havlir, a UCSF infectious disease physician who coled the study. “That’s a very important question. That can generate immediate action.”
The antigen test used in the study is called the Abbott BinaxNOW test, made by Abbott, the health care firm based in Illinois. Antigen tests and PCR tests are both used to determine whether someone has the coronavirus. Antigen tests detect viral proteins, whereas PCR tests detect viral genetic material through a more complex, expensive and timeconsuming process. Because antigen tests are less sensitive, if someone tests positive using an antigen test, the result must be confirmed by a PCR test.
The study compared the Abbott test to the traditional PCR test, collecting two nasal samples from each participant and running them through both tests, and then comparing the results. The PCR tests identified 26 positive cases, including 15 where the individuals had high levels of virus and were likely infectious. The Abbott antigen test correctly identified 14 out of those 15 cases.
The study also indicates the antigen test generates almost no false positives. This is important because getting a false positive test result can be very disruptive, forcing people to stay home from work or separate themselves from their family for days.
“It doesn’t replace all the other kinds of tests, or PCR tests, necessarily,” said Joe DeRisi, copresident of the Chan Zuckerberg Biohub and a UCSF biochemistry professor, who coled the study. “But what it does is provide a rapid, lowbarrier, quick test and certainly in our assay, detected those samples with high levels of virus quite easily.”
The Abbott test can be completed in 15 minutes and costs $ 5 — significantly faster and cheaper than the way most testing is currently done in the United States. The study was the first time the Abbott test was used in a communitybased setting anywhere in the U. S.
“One of the most important things is how fast you can go from getting someone a test and returning an answer to them,” DeRisi said. “If they have to wait three days and they’re infected, that’s three days they could be walking around infecting people. If you return results in 15 minutes, you can give them the
proper instructions to get them in isolation, thus blocking additional forward transmission of the virus.”
The study has not been peer reviewed, and was posted on a preprint server Nov. 12. It was jointly conducted by UCSF, the San Francisco Department of Public Health, Chan Zuckerberg Biohub and the Latino Task Force. ( Biohub is the research institution funded by Facebook CEO Mark Zuckerberg’s philanthropic organization. The Latino Task Force is a community organization that partners with the city on COVID outreach to Latino residents.)
The testing was done over three days in September at an outdoor plaza by the BART station, and the specimens were collected by health care workers who did nasal swabs on the participants. The PCR tests were processed at Biohub’s lab in San Francisco.
Rapid antigen tests, if found to be accurate in additional studies and deployed widely, could help identify and contain spread in communities. These are situations where most people are probably negative, but it’s critical to catch the small number of positives when those people are most infectious so they can selfisolate and not transmit the virus to others. Antigen testing is used in some professional sports leagues to test athletes frequently, but is not widely used in the community.
“I think this is encouraging for the use of rapid antigen testing for public health interventions like this, where they were screening a population,” said Dr. Ben Pinsky, an associate professor of pathology and infectious diseases at Stanford School of Medicine.
Pinsky, who was not involved in the study, said the most important finding was that the researchers came up with a detailed way to interpret the test results, which reduced false positives.
The Abbott test is similar to a home pregnancy test. It has a handheld card that shows one bar to indicate the test has been taken, and a second bar will appear if the person has tested positive. Because the second bar sometimes appears faint or partial — which could produce a false positive result — researchers discussed this issue with Abbott scientists and decided someone was positive only if the bar extended the entire width of the strip. Coming up with this specific way to interpret the results could help others who use the test correctly interpret their results.
“You can improve and reduce the false positive rate by adding these more detailed interpretation criteria,” Pinsky said.
The FDA only allows the Abbott test to be used in symptomatic people within the first seven days of symptom onset. So for it to be used for community surveillance, regulators would have to allow it to be used in asymptomatic people as well. The test must be done by a doctor, nurse or other health care professional, not at home.
It is one of a few rapid antigen tests that the FDA has authorized for the coronavirus, and the one that is poised to be most widely used. This is because the federal government has purchased 150 million of the Abbott tests and is distributing them to states, local health departments, schools and nursing homes around the country. And unlike other rapid antigen tests, the Abbott test also does not require a separate machine to read the results.
San Francisco has received tens of thousands of the Abbott test from the state. Health officials are weighing what settings they might work best in, said Dr. Susan Philip of the San Francisco Department of Public Health.