Procedures for testing for active COVID-19 infection currently rely on obtaining swab samples from a patient’s nose or throat, or sputum. Difficulties obtaining these samples can lead to questions about their quality and result in false negatives.
Researchers from the University of Leicester are now trialling the use of mask sampling, using low-cost face masks with specialized 3D printed inserts which can trap exhaled SARS-CoV-2, an approach they originally developed to detect TB.
Technology Networks spoke with Michael Barer, Professor of Clinical Microbiology at the University of Leicester, to learn more about the masks, the benefits of obtaining samples in this way, and the difference mask sampling could make to the COVID-19 response.
Anna MacDonald (AM): The mask sampling approach for SARS-CoV-2 detection is adapted from work you carried out on TB. Can you tell us more about this and your findings?
Michael Barer (MB): The work is described in a paper we published in a Lancet Journal in February of this year.
We place a sampling matrix inside ordinary masks and ask people to wear them for specific times then analyse the matrix for trapped microbes. We knew from previous work that most people with TB in their lungs breathe out the bacteria. In the Lancet studies we first showed the patterns with which hospital TB patients breathed out the bacteria over a full day and related that to their coughs and the bacteria in their coughed-up sputum. We found that most of these 24 patients consistently breathed out several thousand TB bacteria per hour. Two of our patients breathed out a lot more and two produced none. We were surprised to find that the amount of bacteria detected in masks did not relate to how much people coughed. We even found that several patients breathed out TB bacteria when they were asleep and without coughing at all.
In a second study we showed that our new sampling matrix allowed us to detect TB in people who were sputum negative, effectively several weeks before conventional tests confirmed the diagnosis.
AM: What are the 3D printed strips made of? How do they trap the virus particles?
MB: They are made from a proprietary preparation of Polyvinyl alcohol (PVA). When they are moist, microbes are trapped in the matrix and, when the strips dry out, they are trapped and preserved. There appears to be no need for special storage before analysis, they can be posted to the lab.
AM: How are the contents of the strips subsequently analysed?
MB: The strips dissolve in water then we isolate the virus genetic material (RNA) and run standard tests (PCR) just like those used on swabs.
AM: What are the benefits of obtaining samples in this way, as opposed to using swabs?
MB: No expertise is required to take the samples and health care workers do not need to be close to the person sampled. There is concern about the quality of swab samples taken by inexperienced staff or those self-administered.
We can also vary the time of sampling. At the moment 30 minutes is our minimum and the sampled person can be doing anything during sampling apart from eating. The mask can be taken off and put back on again. Longer sampling should make for greater sensitivity. We also have some evidence that the sample is more representative of what is down in the lungs in contrast to nasopharyngeal sampling.
Finally we are quantitating what people breathe out. We have evidence in TB that this relates to how infectious they are and will soon submit this for publication. Measuring differences in infectiousness and possibly detecting what are called super-spreaders could be very valuable in the post lock-down period.
AM: How soon can SARS-CoV-2 be detected by mask sampling? How does this compare to other methods?
MB: The time scale is similar to swab testing and limited only by test (PCR) availability. The fastest available system could give a result in less than an hour after the mask has been worn. Availability of this kind of testing is very limited.
AM: What further steps are needed before we could see widespread use of the masks as part of testing strategies?
MB: We first need to compare mask results with swab results and have only just got to the point where we can do this. If the mask proves more sensitive we will work with the NHS and PHE to make it widely available. We should know in 3-4 weeks how mask and swab are related.
AM: What difference could mask sampling make to the response to the pandemic?
MB: I think mask sampling could be more easily delivered to large populations than swabbing. I think it could identify highly infectious individuals and allow us to focus on their contacts. Finally, I think the approach could potentially be used to show when people are no longer infectious so that they can return to work.
Michael Barer was speaking to Anna MacDonald, Science Writer, Technology Networks.