Rapid Coronavirus Spit Tests Aren’t Coming Soon

For months, public health experts have been eagerly watching the companies developing spit tests for the coronavirus that could be used at home, producing results in a matter of minutes.

If these rapid saliva tests worked, as many news articles have pointed out, they could greatly expand the number of people getting tested. Some experts have even said they could perform as well as a vaccine in curbing the spread of the coronavirus and paving a path back to normalcy.

But so far, the technology is not panning out as some have hoped.

E25Bio and OraSure, two companies pursuing rapid at-home coronavirus tests, have abandoned efforts to use saliva in their products. Their tests, which detect pieces of coronavirus proteins called antigens, will for now rely on shallow nose swabs instead.

“If I was placing a bet — which I am, because I’m leading an antigen-based testing company — I would say it’s going to be very difficult for antigen-based testing to work on saliva samples,” said Bobby Brooke Herrera, an E25Bio founder and its chief executive. The notion that the virus sets up shop in the mouth and produces enough antigen to be picked up by today’s technology, he said, “is far-fetched.”

Spit also differs vastly among people, and can even change over the course of a single day. “We’ve all noticed that there is variable performance,” said Sarah Jung, scientific director of clinical microbiology at Children’s Hospital Colorado.

Both E25Bio and OraSure plan to seek authorization from the F.D.A. to sell at-home antigen tests using nose swabs instead of spit, a technique similar to the one used by the much-talked-about Abbott antigen test that takes about 15 minutes. The E25Bio test would require people to swab their nose, stir the sample into a chemical soup, apply the mixture to a paper strip and wait up to half an hour for bands to appear. E25Bio’s test picks up on about 80 percent of the infections that ultrasensitive laboratory tests detect — the F.D.A.’s bare minimum for a regulatory greenlight. OraSure declined to give any details about its test’s methodology or accuracy.

Saliva does seem to be working when used in laboratory tests known as P.C.R., which look for bits of the virus’s genetic material, or RNA, rather than antigens. P.C.R. tests detect minute amounts of coronavirus RNA, making them far more sensitive than antigen tests. Research teams at Rutgers and Yale have been granted emergency authorization for these spit P.C.R. tests.

At the University of Illinois, some 10,000 of the institution’s in-house P.C.R. tests are performed each day on saliva from students, faculty and staff members — roughly 1 percent of the nation’s daily tests.

Standard P.C.R. tests, however, take hours to run and are subject to shortages of laboratory supplies, such as pipettes and chemicals, often leading to delays in getting results.

Other scientists, like Dr. Zev Williams of Columbia University, are working on variants of rapid saliva tests that, like P.C.R., detect RNA, but don’t require expensive laboratory machines.

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