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July 8, 2020

Can 'smell tests' screen for Covid-19? Here's what the research says.

Daily Briefing

    Many businesses and workplaces are relying on temperature checks to screen for the novel coronavirus, but new research suggests a different—and perhaps surprising—approach: smell tests, Sharon Begley reports for STAT News.

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    The problem with temperature checks

    According to Begley, research is increasingly revealing that relying on temperature checks to detect whether a person has Covid-19, the disease caused by the new coronavirus, "could well join the long list of fumbled responses to the pandemic."

    Evidence has shown that people who are infected with the novel coronavirus but don't develop symptoms of Covid-19 still can transmit the virus to others. As such, checking individuals for a fever, which is a common symptom of Covid-19, might not be effective at detecting whether someone is infected with the new coronavirus, Begley reports.

     Public health experts and officials are beginning to recognize temperature screenings' shortfalls as a tool for detecting coronavirus infection. For instance, David Pekoske, head of the Transportation Security Administration, last week said, "I know in talking to our medical professionals and talking to [CDC] … that temperature checks are not a guarantee that passengers who don't have an elevated temperature also don't have Covid-19."

    Further, Begley reports that "the reverse is also true," as some people with an elevated body temperature might not have Covid-19. For instance, Begley notes that researchers have found the likelihood that a person with a fever has SARS—an infectious disease caused by a different coronavirus—is between 4% and 65%, depending on the prevalence of the disease where the person is located.

    Would a smell test be more effective?

    According to Begley, a growing body of research suggests that testing people for other symptoms of Covid-19—such as whether they've lost their sense of smell, which is commonly reported by people with the disease—might be more effective at identifying people who are infected with the new coronavirus and are experiencing a mild case of Covid-19 or are in the disease's early stages.

    "My impression is that anosmia is an earlier symptom of Covid-19 relative to fever, and some infected people can have anosmia and nothing else," Andrew Badley, leader of a virus lab at the Mayo Clinic, told Begley. "So it's potentially a more sensitive screen for asymptomatic patients."

    In a study published in April, Badley and his colleagues found that people with Covid-19 were 27 times more likely to have lost their sense of smell when compared with non-Covid patients. In comparison, Covid-19 patients were 2.6 times more likely to have a fever when compared with non-Covid patients.

    In a separate study published in April, Carol Yan, an otolaryngologist at UC San Diego Health, found that patients experiencing anosmia were "more than 10 times more likely to have Covid-19 than other causes of infection," Yan said. She added, "Anosmia was quite specific to Covid-19."

    In contrast, Begley reports, "[f]ever … has many possible causes," and "[t]emperature checks will therefore flag more people as potentially infected with Covid-19 than smell tests will."

    However, according to Begley, the "negative predictive value" of smell tests for Covid-19, or the likelihood that a person with a normal sense of smell is infected with the novel coronavirus and contagious, is not yet clear. One analysis of 24 studies found that about 60% of Covid-19 patients had reported having a normal sense of smell, implying that anosmia is far from a universal symptom of the disease, Begley reports. As such, the tests could miss some infected individuals.

    But including smell tests in addition to temperature checks and other Covid-19 screenings could be an effective way to minimize the number of infected persons that slip through the cracks, Yan said. "I hope it will be used as a screening measure for the virus across the world."

    How would smell tests work?

    If companies and other entities do choose to begin using smell tests to screen individuals for the new coronavirus, they likely would present people with different scents and ask people to identify the scents before they're permitted to enter a building.

    According to Begley, the University of Pennsylvania's Smell Identification Test (UPSIT) is the "gold-standard" of smell tests. For the test, which typically takes between 10 and 15 minutes, users release 40 microencapsulated scents—including dill pickle, banana, and cedar—by scratching them with a pencil. The test taker then chooses from one of four possible responses to identify each scent.

    However, experts say a Covid-19 screening test for anosmia would not need to be as extensive, as the test's goal would be to identify whether people can smell at all—not how well they can identify different scents.

    Instead, businesses and other entities could "ask people to smell a scratch-and-sniff card and pick the correct odor out of four choices" or instruct people to smell three different swabs: one with a standard amount of scent, a second with a diminished scent, and a blank swab to detect both diminished and complete loss of smell, Danielle Reed, associate director of Monell Chemical Senses Center, said. "I can see several practical ways … to have people check their sense of smell as a routine matter when entering public areas" (Begley, STAT News, 7/2).

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