COVID-19 vs common cold: How to tell?

While loss of smell and taste is common in patients with COVID-19 and also in those with a bad cold, there are differences between the two illnesses — providing potential indicators to help doctors discriminate between the two.

One key difference is that nasal congestion — a common symptom with cold — tends to be absent in COVID-19 patients. Also, detection of bitter and sweet tastes was particularly impaired in patients with COVID-19.

“We found that smell loss was much more profound in the COVID-19 patents. They were less able to identify smells, and they were not able to identify bitter or sweet tastes. In fact, it was this loss of true taste which seemed to be present in the COVID-19 patients compared to those with a cold,” said study co-investigator Professor Carl Philpott from the University of East Anglia in Norwich, UK.

“This is very exciting because it means that smell and taste tests could be used to discriminate between COVID-19 patients and people with a regular cold or flu.”

In the study, the researchers compared smell and taste function in 30 individuals (mean age 48 years): 10 with COVID-19, 10 with acute cold, and 10 healthy controls — all matched based on age and sex. [Rhinology 2020;doi:10.4193/Rhin20.251]

Smell function, as assessed using the Sniffin’ Sticks test battery, was significantly poorer in COVID-19 patients vs those with acute cold (identification score, 8.4 vs 12.4; p=0.015).

Moreover, the identification score was able to discriminate between the two groups of patients with 100 percent sensitivity and 80 percent specificity (p<0.001), based on a cut-off value of ≤10.    

In addition, taste function of COVID-19 patients was also significantly worse than those with acute cold, as indicated by the poorer global gustatory score (p=0.015) as well as both sweet and bitter scores (p=0.026 and p=0.001, respectively), assessed using “taste strips”.

In particular, the bitter stimulus showed excellent ability to discriminate between the two groups of patients, with a sensitivity of 90 percent and specificity of 80 percent.

“There are altogether different things going on when it comes to smell and taste loss for COVID-19 patients, compared to those with a bad cold,” said Philpott. “This result shows that taste disturbances reported by COVID-19 patients appear to reflect an impairment in gustatory abilities, and not purely a retronasal olfactory dysfunction.”

"It has previously been suggested that the COVID-19 virus affects the central nervous system, based on the neurological signs developed by some patients,” he explained. “Our results reflect, at least to some extent, a specific involvement at the level of central nervous system in some COVID-19 patients.”

The high sensitivity and specificity of such tests indicate that they could be used as screening tools to identify potential COVID-19 patients.

“Although such tests could not replace formal diagnostic tools such as throat swabs, they could provide an alternative when conventional tests are not available or when rapid screening is needed — particularly at the level of primary care, in emergency departments or at airports,” pointed out Philpott.

“It is particularly interesting that COVID-19 seems to particularly affect sweet and bitter taste receptors, because these are known to play an important role in innate immunity,” he noted.

“More research is needed to see whether genetic variation in people’s bitter and sweet taste receptors might predispose them to COVID-19, or conversely, whether COVID-19 infection changes how these receptors function, either directly or through a cytokine storm,” Philpott suggested.