IN THE UK, essential workers are now among those being sent home testing kits for coronavirus. This involves swabbing the inside of your own nose and the back of your throat, but how useful are the results?
Studies from early in the outbreak in China have suggested that swabs taken by healthcare professionals may give a 30 per cent “false negative” rate, where infected people are told they don’t have the virus (NEJM, doi.org/ggmzsp; medRxiv, doi.org/dvfr). This has prompted claims that self-testing will give even more false negatives and could raise the risk of infected people spreading the virus.
No test is perfect – swabbing technique and analysis errors can lead to inaccurate results. There is no defined false negative level at which covid-19 tests become worthless. “It depends what question you’re asking,” says Graham Cooke at Imperial College London.
On a national level, false negatives matter less, as testing can still give a useful indication of the rates and levels of infection, providing the false negative rate isn’t too high. False negatives are more of a concern at the individual level. In a hospital setting, if someone tests negative for coronavirus but is showing the symptoms, doctors will weigh up whether they think the person should still be placed in a covid-19 ward. “If you’re confident someone’s got covid, you would still ignore a negative,” says Cooke.
However, false negatives in infected but symptomless people are more of an issue, as they may encourage changes in behaviour that spread the virus. If trained healthcare workers get a 30 per cent false negative rate when administering tests, how bad might self-testing be?
There is reason for optimism. Yi-Wei Tang at Cepheid, a diagnostics company in California, says the false negative rate of around 30 per cent recorded early in China’s outbreak may have been higher than it is now. For instance, he says, throat swabs were initially recommended. We now know these aren’t as effective as nasal swabs.
A more recent study in the US suggests self-swabbing is relatively effective. Researchers asked about 500 clinic patients with flu-like symptoms to self-swab their nostrils and their tongues. The results were compared with swabs taken by healthcare professionals from where the back of the nose meets the throat.
The professionals detected more positive results, but the self-swabbers were within 10 per cent of the professional positives (medRxiv, doi.org/ggr7f6). Other types of testing may be a better option. A study that asked participants to drip saliva into collection tubes found that this was a better source of viral material than samples from where the nose meets the throat (medRxiv, doi.org/ggssqf). The false negative rate appears to be lower too – only 12 per cent, compared with 24 per cent for traditional swabbing, says Anne Wyllie at the Yale School of Public Health, who led the study.
Last week, the US Food and Drug Administration authorised testing of home-collected saliva, but studies of saliva testing have yet to appear in peer-reviewed academic journals.
The UK isn’t yet turning to saliva testing. “We are aware of these tests and are awaiting peer-reviewed evidence,” a UK Department of Health and Social Care spokesperson told New Scientist.
newscientist.com, 13 May 2020