People with diabetes, for instance, may have breath that smells fruity or sweet. The odor is caused by ketones, chemicals produced when the body begins to burn fat instead of glucose for energy, a metabolic state known as ketosis.
“The idea that exhaled breath could hold diagnostic potential has been around for some time,” Dr. Davis said. “There are reports in ancient Greek and also ancient Chinese medical training texts that reference a physician’s use of smell as a way to help guide their clinical practice.”
Modern technologies can detect more subtle chemical changes, and machine learning algorithms can identify patterns in breath samples from people with certain diseases. In recent years, scientists have used these methods to identify unique “breathprints” for lung cancer, liver disease, tuberculosis, asthma, inflammatory bowel disease and other conditions. (Dr. Davis and her colleagues have even used V.O.C. profiles to distinguish among cells that had been infected with different strains of flu.)
Before Covid hit, Breathomix had been developing an electronic nose to detect several other respiratory diseases. “We train our system, ‘OK, this is how asthma smells, this how lung cancer smells,” said Rianne de Vries, the company’s chief technology and scientific officer. “So it’s building a big database and finding patterns in big data.”
Last year, the company — and many other researchers in the field — pivoted and began trying to identify a breathprint for Covid-19. During the virus’s initial surge in the spring of 2020, for instance, researchers in Britain and Germany collected breath samples from 98 people who showed up at hospitals with respiratory symptoms. (Participants were asked to exhale into a disposable tube; the researchers then used a syringe to extract a sample of their breath.)
Thirty-one of the patients turned out to have Covid, while the remainder had a variety of diagnoses, including asthma, bacterial pneumonia or heart failure, the researchers reported. The breath samples from people with Covid-19 had higher levels of aldehydes, compounds produced when cells or tissues are damaged by inflammation, and ketones, which fits with research suggesting that the virus may damage the pancreas and cause ketosis.
The Covid patients also had lower levels of methanol, which could be a sign that the virus had inflamed the gastrointestinal system or killed the methanol-producing bacteria that live there. Those breath changes combined “give us a Covid-19 signal,” said Dr. Thomas, a co-author of the study.