Since the emergence of the novel coronavirus SARS-CoV-2, scientists, physicians and key workers have all been calling for more rapid diagnostic testing. A new study from Imperial College London details a device that can give test results in just 90 minutes.
According to the new paper, published in The Lancet Microbe, the new testing kit, CovidNudge, was able to closely match time-consuming laboratory test results, raising hope that anxiously waiting several days for COVID-19 test results, at least in hospital settings, could soon be a thing of the past.
Testing of CovidNudge began back in April, when Imperial researchers led by Professor Graham Cooke started trawling the wards of three hospitals spread across London and Oxford for noses and throats to swab.
Their efforts obtained 386 samples from three groups: self-referred health care workers with suspected COVID-19, emergency department patients with suspected COVID-19 and hospital inpatients admissions with or without suspected COVID-19.
Paired samples meant that Cooke’s group could directly compare the accuracy of their 90-minute test with that of laborious centralized lab tests. The results, as Cooke explained in a press release, were remarkable: “These results suggest the test, which can be performed at a patient’s bedside without the need to handle any sample material, has comparable accuracy to standard laboratory testing. Many tests involve a trade-off between speed and accuracy, but this test manages to achieve both.”
PCR for COVID-19 testing
Currently, lab-based reverse transcriptase polymerase chain reaction (RT-PCR) tests are the gold standard for detecting whether or not someone currently has COVID-19. In such tests, samples are analyzed for viral RNA. If present, the SARS-CoV-2 genetic material is then isolated using an extraction kit and a complimentary DNA (cDNA) copy is made from the viral RNA template. Primers amplify these sequences to enable the virus' genetic material to be detected.
A note on “antigen” testing
RT-PCR tests, such as those done in labs and in the CovidNudge device, have colloquially been called “antigen tests” in COVID-19 reporting.
Technically, true antigen tests are a different kind of test that look at viral proteins rather than genetic material – these types of tests are not yet widely available for COVID-19. PCR testing is more accurate, although far more time-consuming. Problems with sample processing and test availability have recently swamped health services in the UK, with just one-third of PCR tests being turned around in 24 hours in England during the week to 9 September.
Enter CovidNudge. The team had identified that an ideal rapid COVID-19 diagnostic would require no sample pre-processing whatsoever to simplify the test. CovidNudge is, essentially, a portable PCR platform spread across two devices:
- A 40g blue container, called the DnaCartridge, which looks a bit something you might put your retainer in at night. A nose and throat swab is taken from a patient and inserted directly into the DnaCartridge. The sample preparation is entirely automated within this cartridge.
- The cartridge is then introduced into the NudgeBox processing unit, which weighs 5kg and is roughly the size of a shoebox. This contains all the testing equipment required to run a real-time RT-PCR test.
One additional feature of CovidNudge is its inclusion of a sample adequacy control which verifies that the sample has been captured on the swab. This control is a human gene known as RNaseP. In some other portable tests, the lack of an adequacy control has opened up the possibility of false negative results.
Sensitivity and specificity
So how did CovidNudge perform? The test achieved an overall sensitivity of 94%, and a specificity of 100% compared with lab-based tests. Sensitivity measures the likelihood that a COVID-19 positive sample will test positive, whereas specificity measures how likely a negative sample will test as negative. Of the 386 samples that were collected, CovidNudge provided 67 positive results, as opposed to 71 positive results from the lab test.
In comment to the Science Media Centre, Lawrence Young a professor of molecular oncology at the University of Warwick, who wasn’t involved with the research, said, “The CovidNudge test, like other point-of-care diagnostics, could have an important role where near-patient real-time decision making is necessary such as screening patients for admission to hospital or for surgery.”
Despite this potential use, Paul Hunter a professor in medicine at the University of East Anglia said the device’s low-throughput – it can only measure one sample at a time – meant that it would probably not fill the need for rapid mass-testing. Furthermore, Hunter queried whether the promising accuracy results could be repeated in non-clinical settings: “Although the sensitivity and specificity look very good this is based on initial testing in centers of excellence. Whether this degree of accuracy could be maintained during routine use in the real world is a big question and needs to be monitored during any roll out of the technology.”
For now, CovidNudge remains a clinical tool but has now been installed in eight London hospitals. DnaNudge’s CEO and co-founder, Imperial Regius Professor Chris Toumazou, commented: “The DnaNudge test was developed as a lab-free, on-the-spot consumer service that can be delivered at scale, so we clearly believe it offers very significant potential in terms of mass population testing during the COVID-19 pandemic. The platform is well suited to testing in primary care and community settings with potential for use in non-healthcare settings such as care homes, schools, transport hubs, offices, and, to help bring the arts back, in theatres and venues.”