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Workplaces do it. Newly reopened public libraries do it. LAX does it. Some restaurants, bars, and retail stores started doing it when governors let them serve customers again: Use temperature checks — almost always with “non-contact infrared thermometers” — to identify people who might have, and therefore spread, the infectious disease.

Unfortunately, temperature checks could well join the long list of fumbled responses to the pandemic, from the testing debacle to federal officials’ about-face on masks.

Because many contagious people have no symptoms, using temperature checks to catch them is like trying to catch tennis balls in a soccer net: way too many can get through. On Tuesday, the head of the Transportation Security Administration told reporters, “I know in talking to our medical professionals and talking to the Centers for Disease Control … that temperature checks are not a guarantee that passengers who don’t have an elevated temperature also don’t have Covid-19.” The reverse is also true: Feverish travelers might not have Covid-19.

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In this case, however, a growing body of science suggests a simple fix: make smell tests another part of routine screenings.

Of all the nose-to-toes symptoms of Covid-19, the loss of the sense of smell — also known as anosmia — could work particularly well as an add-on to temperature checks, significantly increasing the proportion of infected people identified by screening in airports, workplaces, and other public places.

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“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,” said physician Andrew Badley, who heads a virus lab at the Mayo Clinic. “So it’s potentially a more sensitive screen for asymptomatic patients.”

In a recent study, Badley and colleagues found that Covid-19 patients were 27 times more likely than others to have lost their sense of smell. But they were only 2.6 times more likely to have fever or chills, suggesting that anosmia produces a clearer signal and may therefore be a better Covid-catching net than fever.

There is no definitive study on the predictive value of temperature checks for Covid-19. But there are clues from when that strategy was used during the SARS epidemic of 2003. Deployed at airports, especially in Asia, the devices fell far short of the ideal, an analysis found. Although contactless thermometers are quite accurate if used correctly, many other conditions (including medications and inflammatory disease) can cause fever. As a result, the likelihood that someone with a fever had SARS ranged from 4% to 65%, depending on the underlying prevalence of the disease.

The likelihood that someone with a normal temperature reading was SARS-free was at least 86%. That suggests SARS fever checks didn’t miss many infected people. Unlike SARS, unfortunately, Covid-19 can be contagious even before an infected person runs a fever, which makes missed cases more likely.

As experts have cast around for other screening tools, some have zeroed in on smell tests, which could be as simple asking people to identify a particular scent from a scratch-and-sniff card. Though not a universal symptom, loss of smell is one of the earliest signs of Covid-19 because of how the virus acts. Support cells in the olfactory epithelium, the tissue that lines the nasal cavities, are covered with the receptors that SARS-CoV-2 uses to enter cells. They become infected very early in the disease process, often before the body has mounted the immune response that causes fever.

“These support cells either secrete molecules that shut down the olfactory receptor neurons, or stop working and starve the neurons, or somehow fail to support the neurons,” said Danielle Reed, associate director of Monell Chemical Senses Center, a world leader in the science of taste and smell. As a result, “the [olfactory neurons] either stop working or die.”

In an analysis of 24 individual studies, with data from 8,438 test-confirmed Covid-19 patients from 13 countries, 41% reported that they had lost their sense of smell partly or completely, researchers reported in Mayo Clinic Proceedings. But in studies that used objective measurements of smell rather than simply asking patients, the incidence of anosmia was 2.3 times higher.

A Monell analysis of 47 studies finds that nearly 80% of Covid-19 patients have lost their sense of smell as determined by scratch-and sniff tests, Reed said. But only about 50% include that in self-reported symptoms. In other words, people don’t realize they have partly or even completely lost their sense of smell. That may be because they’re suffering other, more serious symptoms and so don’t notice this one, or because smell isn’t something they focus on.

In a recent study of 1,480 patients led by otolaryngologist Carol Yan of UC San Diego Health, someone with anosmia was “more than 10 times more likely to have Covid-19 than other causes of infection,” she said. Nasal inflammation from some 200 cold, flu, and other viruses can cause it, she said, but especially during the summer, when those infections are pretty rare, the chance that anosmia is the result of Covid-19 rises.

“Anosmia was quite specific to Covid-19,” she said.

Fever, in contrast, has many possible causes. Temperature checks will therefore flag more people as potentially infected with Covid-19 than smell tests will. The likelihood that anosmia indicates Covid-19, called a test’s positive predictive value, increases as the prevalence of Covid-19 increases, as it is in many areas of the U.S.

A key unanswered question is a smell test’s “negative predictive value”: If someone has a normal sense of smell, the chance that he or she is nevertheless infected and likely contagious. Because at least some people infected with SARS-CoV-2 will have a normal sense of smell, especially early on, even experts who believe that anosmia screening can be widely beneficial — “I hope it will be used as a screening measure for the virus across the world,” Yan said — say it should be added to fever checks or other screening tools, but shouldn’t replace them.

“There is value in evaluating anosmia screening as a way to identify asymptomatic spreaders,” said Badley, the Mayo Clinic researcher.

UC San Diego Health is doing that. It asks about loss of smell (and taste) when it screens visitors and staff before allowing them to enter its buildings.

Because many people are unaware of their anosmia, testing would be even better than asking, Reed said.

The gold-standard test is the University of Pennsylvania Smell Identification Test, called UPSIT. It uses 40 microencapsulated scents — including dill pickle, turpentine, banana, soap, licorice, and cedar — released by scratching with a pencil. The test taker has a choice of four answers for each, and the whole thing takes 10 to 15 minutes.

A screening test for anosmia in the context of Covid-19 could be much simpler, experts say, especially since the idea is to identify whether individuals can smell or not, rather than whether they can discriminate different scents.

“I can see several practical ways is to have people check their sense of smell as a routine matter when entering public areas,” Reed said. Medical offices could “ask people to smell a scratch-and-sniff card and pick the correct odor out of four choices. For workplaces and schools, one way is to ask people to ‘stop and smell the roses’ as they enter buildings and report abrupt reductions in their ratings of odor intensity.”

To avoid cultural bias (not everyone knows what bubblegum or grass smells like), a test for anosmia in Covid-19 could have a standard amount of phenyl-ethyl alcohol (which smells like roses) on a swab or stick and have people sniff it, Reed said. A second stick could have less, testing for diminished sense of smell. A third stick could be a blank, to identify people who falsely claim they can smell.

  • The utility of infrared measurement of body temperature might well be more useful if the infrared beam were pointed into the open mouth rather than at the forehead. Forehead temperature differs systematically from mouth temperature, so the latter may well be a better early warning system. There need be no concern about a testee exhaling through the mouth during the test. That tends not to happen. As most of us recall from our childhood, to get us to open the throat the doctor always had to prompt us with “Now say AHHH!”

    • I’ve heard it said that people don’t really get a fever from the virus, especially early on. Maybe a low grade temperature, but the temp comes up if you get a secondary bacterial infection, so it’s not reading on the viral infection, but the bacterial infection. It could also read on any bacterial infection. Using temperature is just another silly thing the “experts” are trying to trick us with.

  • Low or absent sense of smell is fairly widespread, and it is well known that the ability to discern scents varyies a lot among people. This sounds like yet another bad idea… yet another ineffective measure (against a threat that may have been significantly overblown, as another article on Stat warned: https://www.statnews.com/2020/03/17/a-fiasco-in-the-making-as-the-coronavirus-pandemic-takes-hold-we-are-making-decisions-without-reliable-data/ )

    More food for thought:
    https://off-guardian.org/2020/06/12/study-80-of-people-naturally-resistant-to-coronavirus/

    Sensible steps to protect the most vulnerable – particularly the elderly with one or more health issues – WITHOUT obliterating the basic rights of billions of people, with real harm to the lives – and health – of countless millions that could have been avoided. Economic hardship for individuals and families,m and deep damage to our economy (while a handful of mega-corporations profit massively)… Unemployment at higher rates than the USA has seen since the Great Depression… Homelessness increasing… Despair, depression, and suicide increasing… Other health issue increasing as the stress of being treated like prisoners (‘lockdown’ is a prison term), and being unable to get outside. or be in nature, or exercixe… Even wearing masks for extended periods can reduce the fresh air and oxygen we breathe, and cause other harms: links to references included in this article: https://articles.mercola.com/sites/articles/archive/2020/06/19/do-face-masks-help-against-coronavirus.aspx

    Those who wish to live inside a plastic bubble are free to do so.

  • Interesting article.
    50 million Americans suffer from allergies and thus regularly experience “symptoms” attributed to covid19. In my case I know sense of smell is impaired. I don’t think any one “test” will fit all people. There is too much variability between people! I’ve felt all along that temperature checks are more about making the general population “feel” safe than actually protecting anyone.

  • Sharon,
    Read your article on Asomia and Covid-19. No where did you account for the many people in their “insensitivity” to smell have that as common loss that comes with aging nor those of us who have drug induced loss particularly due to steroid use. This is common but I lost my smell years ago. How do researchers factor for this. It disturbs me that I might be considered positive when in fact its is not an adequate Guage for me whether I might have asymptomatic Covid-19. I would be upset if that was a gage for determining my status.
    I think you will find this common in people with autoimmune diseases which Covid-19 has many similar symptoms

    • I agree, those with many types of autoimmune suffer nasal issues. I have smell and taste issues with my autoimmune disease of 5yrs. Under normal circumstances. Like I try to explain to those I know, many autoimmune diseases make you feel like you have flu symptoms 24/7/365 if you’re experiencing a flare

  • I agree with previous comments pointing out a diminished sense of smell because of age, allergies, or medication. Add to that head injuries (concussions) and gender; men have been shown to have a diminished sense of smell.

  • Another cause of ability to smell is repeated use of allergy medications. I’ve experienced this during bad pollen seasons. That’s not a good reason for using the smell test though since it is more accurate than the temperature taking.

  • Interesting approach but… as a person who has for decades had random but reasonably frequent bouts of anosmia, I can foresee a near future of being denied access to flights, restaurants, shops etc because nobody will believe that my anosmia is not covid-19 related. I understand mine may be a relatively rare problem, and if this test really contributes to the greater good, so be it. But a screen that may be a bit more reliable than one that is completely unreliable is probably not what the world is looking for.

    • I’m in the same boat as you, Tetsu. I understand that smell tests are a good idea, but when anosmia strikes, I don’t want to barred from entry to the store or the library. I don’t intimately know the state of testing in the United States for covid-19 at the moment, but if more tests become available, some of them could certainly be offered to people who cannot satisfy more sophisticated screenings can check for covid-19, even if they aren’t yet displaying symptoms.

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