Loss of smell, taste highly predictive of COVID-19

Loss of sense of smell (anosmia) and taste (ageusia) are highly prevalent among COVID-19 patients, especially early on in the disease, suggesting that these symptoms may be indicative of an infection with the coronavirus, according to recent studies.

“Based on our study, if you have smell and taste loss, you are more than 10 times more likely to have COVID-19 infection than other causes of infection,” said Dr Carol Yan from UC San Diego Health, San Diego, California, US, lead author of a study on anosmia in COVID-19 patients.

“The most common first sign of a COVID-19 infection remains fever, but fatigue and loss of smell and taste follow as other very common initial symptoms,” she explained.

Yan and her team found that loss of smell and taste was highly prevalent among patients with COVID-19 vs those tested negative for the disease (p<0.001). Anosmia was present in 68 percent of the infected patients compared with 16 percent of COVID-negative patients. [Int Forum Allergy Rhinol 2020;doi:10.1002/alr.22579]

Similarly for loss of taste, the symptom was experienced by 71 percent of COVID-positive patients, as opposed to 17 percent of the patients who tested negative for COVID-19.

Importantly, both smell and taste impairment were strongly and independently associated with COVID-19 infection (adjusted odds ratio [aOR], 10.9, 95 percent CI, 5.08–23.5 for anosmia and aOR, 10.2, 95 percent CI, 4.74–22.1 for ageusia).

In contrast, participants who reported having a sore throat more often turned out to be negative for COVID-19 (aOR, 0.23, 95 percent CI, 0.11–0.50).

Also, sensory return was typically consistent with the timing of disease recovery. Among the 38 COVID-19 patients who experienced loss of smell, 74 percent had resolution of anosmia with recovery from the disease.

“Our study not only showed that the high incidence of smell and taste is specific to COVID-19 infection, but we fortunately also found that for the majority of people sensory recovery was generally rapid,” Yan said. “Among the COVID-19 patients with smell loss, more than 70 percent had reported improvement of smell at the time of survey and of those who hadn't reported improvement, many had only been diagnosed recently.”

More predictive than fever for COVID-19?

“We believe that loss of smell and taste is a very common COVID-19 symptom and in fact, occurs more often than fever and lasts longer,” said Professor Tim Spector from King's College London, UK, who led an observational study of 76,260 people in the UK who reported symptoms on the COVID Symptom Study app. [Lancet 2020;doi:10.1016/S0140-6736(20)31281-2]

“We found that the median duration of anosmia symptoms was 5 days, whereas the median duration of fever was only 2 days,” the researchers added.

Among the 13,863 individuals who tested positive for COVID-19, 64.5 percent had experienced loss of smell and taste overall and more than a quarter (28.5 percent) never showed fever or cough.

Furthermore, 15.9 percent of COVID-19 patients who did not have fever or cough reported a loss of smell and taste — indicating that quantifying additional symptoms for COVID-19 could save more lives that would have been missed by the absence of conventional symptoms like fever and cough.

Further analysis of the sensitivity and specificity of the respective symptoms revealed that loss of smell and taste was more predictive of COVID-19 than fever or persistent cough.

“However, loss of smell and taste could be a less discriminating symptom in older people and by those who are less able to report symptoms, such as people in care homes,” cautioned Spector and co-authors.

Anosmia without nasal block a red flag

A separate systematic review of 19 studies came to the same conclusion that anosmia, especially one that occurs without nasal obstruction, may be indicative of COVID-19. 

One of the studies included in the review showed that among 55 patients who presented with anosmia but not nasal obstruction, 94 percent tested positive for COVID-19.     

“The occurrence of sudden onset anosmia without nasal obstruction is highly predictive of COVID-19 and should trigger the individual to immediately self-quarantine with presumptive COVID-19 [and further testing],” highlighted Dr Ahmad Sedaghat from the University of Cincinnati Medical Center, Cincinnati, Ohio, US, who led the systematic review. [Otolaryngol Head Neck Surg 2020;doi:10.1177/0194599820929185]

Further evidence ensued from additional independent studies. One study involving 72 outpatients with COVID-19 revealed that 74 percent had anosmia, 69 percent reported dysgeusia, and 68 percent of them experienced both symptoms. Another retrospective study of 2,013 COVID-19 patients found anosmia in 87 percent and taste impairment in 56 percent of the patients, respectively. [Clin Infect Dis 2020;doi:10.1093/cid/ciaa525; Ann Intern Med 2020;doi:10.7326/M20-2428]

Following these discoveries, the AAO-HNS* has recommended that anosmia be considered as a symptom to screen for COVID-19. Also, the US CDC has now added anosmia into its list of COVID-19 symptoms.  

“Anosmia, in particular, has been seen in patients ultimately testing positive for the coronavirus with no other symptoms,” according to a statement from the AAO-HNS.

“COVID-19 is not associated with the symptoms that are typically associated with a viral cold such as nasal blockage or mucus production,” said Sedaghat. “This distinction is also why it is fairly easy to distinguish COVID-19 from seasonal allergies.”

“The sinonasal tract may be an important site of infection while sinonasal viral shedding may be an important transmission mechanism,” he proposed.  

Taste dysfunction as well?

In addition to anosmia, another meta-analysis of 817 patients found that almost half (49.8 percent, 95 percent confidence interval [CI], 8.2–91.5) had experienced changes in sense of taste. [Gastroenterology 2020;doi:10.1053/j.gastro.2020.05.003]

“Earlier studies didn't note this symptom, and that was probably because of the severity of other symptoms like cough, fever and trouble breathing,” said lead author Dr Muhammad Aziz from the University of Toledo in Toledo, Ohio, US.

“We propose that this symptom should be one of the screening symptoms in addition to the fever, shortness of breath and productive cough. Not just for suspected COVID-19 patients, but also for the general population to identify healthy carriers of the virus,” he suggested.