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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.

  • When I had sinus surgery 2 decades ago, I lost all sense of smell for many months. The doctor told me it will eventually come back, and it did. The point is – the Wuhan Cold isn’t the only thing that can make you smuff.

  • I can’t tell what scent those scratch and sniff things are anyway, how is this any better than taking temperature? Having been an educator I know full well parents give their kids Tylenol to cover up their fever, send them to school, and after a few hours they are in the nurses office going home with a fever. Not hard to cover up a fever, people.

  • What about those folks who already don’t have a well-functioning sense of smell? I know several older folks who have lost that ability, and a few younger.

  • I have already had Covid-19, but three months later still have symptoms, not fever, but definitely loss of smell. Without an immunity certificate i would be refused a flight or put into two weeks quarantine.
    Yes loss of smell is the most accurate predictor, see the Zoe study (over 3 million contributors) . However i had a recent covid-19 test which was negative (r PCR swab test, UK health service) they do not offer or encourage Antibody testing. Yet the whole rush for a vaccine is exactly that… antibodies. Scientific studies are showing some cities over 20% immune (by post infection). As perhaps half of live cases are completely asymptomatic I would suggest that smell loss screening is useless as
    :
    a; it punishes those who are immune and thus unable to transmit covid
    b; it misses most of the infected-and- infective cases

  • Smell test needs to be self-administered. People won’t want to remove their mask and sniff unless they feel secure. Just give them two scratch off cards each with one of 6 odors and a reply card where they indicate what they smelled. Chance of guessing both right is quite small.

  • using temperature tests as a pass is INSANE. I am not surprised, however, that the CDC is allowing it. Redfield is way over his head managing the CDC and a Trump tool. If they think this con will allow business to open up then CDC will encourage it. Simple enough to test the value of temperature-taking as a preventative. At a testing site, take the temperature along with a sample from every individual. I suspect the results will be TERRIFYING. For every one individual stopped due to fever, probably hundreds of infectious people pass through. DEMAND the data from the CDC.

    • Temperature checks are done because they are easy and quick (5 seconds), not because they are particularly effective. They figure if they catch some sick people, that is good enough.

  • There is a problem with relying on loss of smell. In 2004 I had a bout with Ramsey Hunt syndrome. In addition to the facial paralysis and hearing loss in the affected ear, I also lost all sense of smell. I am awaiting the results of the drive up test at a local CVS.

  • It might not be a good test for mass screening of the public, but it might be a good in-home test in conjunction with taking temperature for reassuring the worried well. I soon as I read the article, I remembered smelling the shrimp in the salad I had for breakfast earlier this morning, and then I sniffed the applesauce I had for a snack. I could smell the apples. I live in Santa Clara County (a COVID-19 hotspot of California), and there was an announcement on TV asking the public not to call 911 except in real emergencies because of the load on the system. If this can avoid a few phone calls, it could be valuable.

  • This sounds like an interesting idea, however we must consider that anosmia is a common condition in people with Parkinson’s Disease and the incidence of PD in the population is not insignificant, leading to a potentially high proportion of false positives (especially where incidence of COVID is low). Sounds great but we must remember to account for how this will impact people with mobility issues.

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