False negatives in particular have been a problem with the nasopharyngeal swabs. (A different type of test for antibodies, which can say whether a person was exposed to the virus and has recovered, is riddled with false positives.)
In separate research, a Yale University team reported that saliva may be able to detect the virus in people who are only mildly ill, while a nasopharyngeal swab cannot.
In their study, the team compared swabs and saliva samples from patients. They needed only a few drops of saliva for their test, an advantage for people who may have trouble producing more. Thinking about a favorite meal can often do the trick, said Anne Wyllie, the Yale team’s leader.
The swabs are known to produce false negatives — perhaps in part because of errors by health care workers under stress. The saliva test appeared to be more consistent and accurate over a longer period of time, detecting infections even after the amounts of the virus have waned, than the swab.
“The nasopharyngeal swab is subject to so much more variability in how well it’s obtained,” Wyllie said. A saliva test is “definitely more reliable.”
In one case, the team found a health care worker who twice tested negative using a nasopharyngeal swab before finally testing positive on a third day. But the worker’s saliva tested positive all three days, Wyllie said. She underlined the risks of asymptomatic health care workers getting a false negative and continuing to care for patients. “You can imagine the implications,” she said.
While the Yale team did not compare saliva tests with the shorter swabs used in some tests, Wyllie said she expected that saliva tests would prove superior there as well. Most people with COVID-19 do not have runny noses, which might influence how much virus a short swab can collect, she said.
Saliva tests would also be a preferred choice for at-home tests, Adalja added. A saliva test for HIV is the only at-home test approved for an infectious disease, he said, but before the pandemic, the federal Biomedical Advanced Research and Development Authority had funded two companies to develop at-home nasal swab tests for influenza.
“It’s not a high bar to repurpose home testing for the coronavirus,” he said. “It’s not something that’s out of reach.”
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