The amount of coronavirus found on the nasopharyngeal swab tells us very little about how much of the virus is actually emitted into the surrounding area. This is the conclusion of a pilot study that also examines the viral load of exhaled air.
The procedure has become routine at the test centre: with the head of the person being tested tilted slightly backward, the cotton swab is pushed slowly along the nasal septum deep into the nose to the pharyngeal wall and twisted back and forth as it is removed. These nasopharyngeal swabs are the current reference method used to take samples for PCR evidence of SARS-CoV-2. Madiha Malik from the Institute of Pharmacy has examined another method of providing evidence by testing exhaled air. "When you are dealing with detecting and interrupting chains of infection and minimising the occurrence of infection, the amount of virus that is actually emitted into the surrounding area is crucial," explained the doctoral researcher in the Clinical Pharmacy department.
In her doctoral work at the Institute of Pharmacy Malik is examining the extent to which air exhaled by a person is diagnostically useful and which parameters can be determined from a person’s breath. Breath can be diagnostically relevant in the same way as a blood sample can contain important indicators. "Hardly any systematic use has been made of this to date," said her PhD supervisor Professor Thomas Kunze, but there are good reasons for doing so, he added. It makes sense, for example, to find evidence of viruses like SARS-CoV-2, which are transmitted through the air, in the air we breathe out, he said. Malik is examining whether this can be done and how it compares with traditional methods in a pilot study of patients who were hospitalised between July and November 2020.
It is not possible to gauge how infectious a person is by swab alone.
The study includes individuals who tested positive for coronavirus but did not have to be treated in the ICU. "We have taken samples from the exhaled air and a nasopharyngeal swab at the same time. Both samples were analysed identically via PCR." The patients were studied for 14 days after their initial diagnoses or for as long as they were in the hospital. Samples were taken once every two days to determine how the amount of virus changed over the course of time. The exhaled air was measured using a device similar to an asthma inhaler. Participants of the study breathed 20 times through a mouth piece. A filter in the device captured the tiny virus particles. These filters were then analysed.
Analysis of the samples revealed that although swabs of the upper respiratory tracts are an effective instrument for COVID-19 diagnosis, they cannot predict exactly how many virus copies are actually exhaled by infected individuals. "The viral load in the swabs was significantly higher than in the samples of exhaled air. But there was no correlation between the nasopharyngeal swab and samples of exhaled air taken at the same time," said Malik. In other words, the amount of virus that an infected person emits into the surrounding area is not reflected in the amount of virus found in the nasopharyngeal swab. What was evident, however, was a very high level of heterogeneity in the amount of virus in the breath. "Some individuals, for example, exhaled just 92 viruses in 20 breaths right at the start of the infection, while others emitted nearly 30,000 viruses at the same point in their infection. The logical conclusion to be drawn from this is that it is not possible to gauge how infectious a person is by swab alone." By contrast, analysis of the amount of virus in exhaled air could help identify superspreaders at an early stage.
The work published in the renowned International Journal of Infectious Diseases only featured a relatively small number of examined individuals. "We are nevertheless convinced that tests of exhaled air are of huge importance to improving how we cope with the challenges of a pandemic like coronavirus. They should therefore be incorporated into infection prevention and infection control measures," stressed Professor Thomas Kunze.
Coronavirus is just one example of where examination of the air we breathe could be useful. "A person’s exhaled air is as individual as a fingerprint," said Malik, who would like to analyse the exhaled air profile of people with other diseases as a next step. This could result in promising opportunities for diagnostics.
Author: Kerstin Nees